# Why small tanks need more watts per gallon



## epicfish (Sep 11, 2006)

Interesting thought, to say the least. I'm going to have to ponder this a bit more.


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## epicfish (Sep 11, 2006)

Hoppy said:


> So, if a 55 gallon tank needs one 54 watt T5 bulb to get adequate light at the substrate, a 10 gallon tank would need about 40 watts of T5 light for the same light intensity.


If one wanted two 54W T5HO bulbs over a 55 gallon tank, it would mean that a 10 gallon tank would need about 74 watts of equivalent T5HO lighting for the same intensity, if my calculations are the same ones you're performing.


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## Hoppy (Dec 24, 2005)

If there were 20 inch T5HO bulbs available, they would be 22 watt bulbs, I think. So, two of those would only give about 27% as much light intensity as the 2 48 inch 54 watt bulbs on a 55 gallon tank. So, you would need 1/.27 times 44 watts, or 162 watts, which is 7 22 watt bulbs. (That doesn't seem right, but that seems to be what that table says.) I don't think I would want to put that much light on a 10 gallon tank.


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## Wasserpest (Jun 12, 2003)

It's an interesting thought, makes sense to me at first. 

What you are saying is that a 10gal tank lit by a 20W bulb is way dimmer than two 10gal tanks lit by a 40W bulb. Not so sure about that.

Now what about multiple bulbs... are two 4ft 28W bulbs brighter then one 4ft 54W bulb? :smile:


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## die2win (Aug 10, 2008)

Re read his post.

Using a single bulb of a fixed length determines the amount of light intensity. A ten gallon tank is considerably shorter than a 55g's 48" so it would need more watts per inch to get the same light to the ground as in the 55g given the light tubes were the same length to tank ratio. :fish:


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## Hoppy (Dec 24, 2005)

die2win said:


> Re read his post.
> 
> Using a single bulb of a fixed length determines the amount of light intensity. A ten gallon tank is considerably shorter than a 55g's 48" so it would need more watts per inch to get the same light to the ground as in the 55g given the light tubes were the same length to tank ratio. :fish:


Good summary! I get confused with this stuff too.

One of the starting points for that table is that all of those tanks have the same type of light bulb, T5HO, for example, with each bulb producing the same amount of light per inch of bulb. Once you change one tank to a different type of bulb, T8 for example, the table means nothing.


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## SpeedEuphoria (Aug 5, 2008)

This is what it comes down to, except I dont see any PAR data


Hoppy said:


> Do a mind experiment: Take a four foot long tank, with a four foot long bulb above it. Now insert a pair of opaque dividers in the tank so that there is two feet between them and only two feet of the bulb lights the substrate between them. Same bulb, same tank, but the light intensity at the substrate drops considerably. That is because that section no longer gets light from the blocked off ends of the four foot long bulb. Move those opaque dividers close together, so they are an inch apart - the substrate in that section is almost dim, but it is still the same bulb up there.


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## Wasserpest (Jun 12, 2003)

One thing to ponder is how much of the tank/which parts actually benefits from that additional light via additional bulb length.

If you have a 24" tank lit with 20W, and a 48" tank lit with 40W, and both have the same height and width, I am going to suggest that both tanks have exactly the same light level in the center and nearby.

Both tanks also have some reduced light levels towards the sides of the tank, where the bulbs end.

If you insert opaque, especially black, dividers then yes, there will be some light reduction. But each tank has only two ends, so adding more ends in the center doesn't really make a lot of sense.

I think that the reason why larger tanks need proportionally less light has to do with the volume vs surface area relation. It has been suggested before that it might be more useful to compare light levels per square inch surface area. But that is slightly more complicated and so we will still see folks asking if 3.165 Watts/gallon is sufficient.


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## snafu (Oct 9, 2004)

if you look at standard tank sizes, you'll find the area per gallon (sq in per gal) significantly higher for small tanks. in some cases 100% or more higher. take the following examples:

if you take a 10 gal (20in x 10in) tank versus a 40 gal (36in x 18in), you'll find the following:

10 gal -> 20 sq-in per gal
40 gal -> 11 sq-in per gal

so in order to put the same intensity (or flux) over the two tanks, you'll need twice the light for the smaller tank.


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## Hoppy (Dec 24, 2005)

Well is time to eat some crow - I do this often enough to have developed a taste for it. My calculations above were made with the assumption that light intensity drops off proportional to distance in the aquarium, which isn't true. For a calculus approach to this, the emitter of the light is infinitestimally small, so the light intensity has to drop with the square of the distance. Redoing those calculation with that assumption gives entirely different numbers. In fact that calculation shows a 10 gallon tank with the same light intensity per inch of bulb as on a 55 gallon tank actually has a higher intensity at the substrate level in the middle of the tank. Oops!

But, that triggered an all day effort to include reflected light from the front and back glass into the calculation. I finally was able to do that, assuming 100% reflection from the glass, which isn't correct, but it is close enough. Again, the 10 gallon tank has the highest intensity at the middle of the substrate level. Then studying the equation I derived I decided that the most significant factor is the depth of the tank, so I redid the calculations using the same 12 inch depth for each tank size (keeping the other dimensions accurate, so each tank became a smaller tank, except for the 10 gallon tank.) This time the 55 gallon tank has the highest intensity, but not by enough of a factor to be very significant. (This would be the intensity at the 12 inch depth of each tank.)

Now I'm convinced that tank depth is the most significant factor in how much light is needed for the tank. But, the variation with depth is still small enough that I can't see a use for the knowledge. One exception is a 90 gallon vs a 55 gallon tank, where the deeper tank really does need more light.









The equation for the intensity at the substrate level in the center of the tank is: Intensity is proportional to 1 divided by the square root of [Depth squared + front to back squared] that multiplied by the arctan of [Tank length divided by {2xthe square root of (depth squared + front to back squared)}]

This has been fun! Now, why does a 10 gallon tank require so much more light than a 55 gallon tank to get a good light intensity for growing plants?:icon_redf

I don't see how this can be related to the substrate area, because the light at the point directly under the center of the bulb is the same for the same bulb no matter how big the substrate area is, and from my experience on a 10 gallon tank that light has to be much greater than for a bigger tank.

Once a lot of us have access to PAR meters and take some time doing measurements, the answer to this question should become obvious, and irrelevant, since we should then be able to pick an appropriate light using just the data we will have.


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## Captain Hooked (Oct 13, 2008)

I always thought it was because most plants needed a certain minimum amount of light intensity to thrive- which coincided with the smaller tank needing more WPG than a larger tank (of equal depth). This sat well in my available head space...don't confuse me with the _facts_! :eek5:


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## stagger (Apr 5, 2006)

Great work Hoppy, this has always been a confusion subject.

I have a 96watt 65K quad PC 20 inch light, would this over a 20 be too much?
I would have Pressurized Co2


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## epicfish (Sep 11, 2006)

Yes.


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## Hoppy (Dec 24, 2005)

seconded


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## eyebeatbadgers (Aug 6, 2007)

Hoppy said:


> This has been fun! Now, why does a 10 gallon tank require so much more light than a 55 gallon tank to get a good light intensity for growing plants?:icon_redf


Does it really though? How much light have you been using on a 10 gallon tank? For these two tanks in particular, one(ish) watt/inch of length from a good quality setup would provide adequate light in either situation, I think. 

A realistic example that could be tested might be a single 24 watt T5 bulb/reflector over a 10 gallon tank, and a single 54 watt T5 bulb/reflector over the 55. Would that be enough light? I think it would, with the 10 gallon tank being a bit brighter, as your second graph shows. Am I on the right track?

Edit: However, when I look at my 29 gallon tank, I can't imagine how slow the growth would be if I cut the lighting to 30ish watts. The growth is fairly slow at just 55 watts, and is rapid at 110. In my 10 gallon tank, with two 13 watt CFL bulbs, plants grow quickly and low to the substrate. Ah, what I'd give for a PAR meter!

Edit again: Clearly, a 29 gallon tank with just 30 inches of light would be less intense than a 55 gallon with 48", because the proportions of the tank are different. The 55 is rationally longer, and the 29 is taller. This just shows that watts/inch of length isn't any better than WPG. Oh well, I thought I was on to something.:icon_redf


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## SleepyOwl (Sep 23, 2007)

Interesting.

It always confuses me when people say you need so much more light in smaller tanks, especially for low light plants. I have no experience with high light, but I have an Eclipse 6 that I have had about 6 years. It has an 8 watt bulb. The bulb is probably 2+ years old now, so not as bright as new, yet I can grow low light plants with no problem. (Anubias, Jave fern and Java Moss) They grow slowly, but they grow, and are healthy. Maybe it is because the 6 gallon tank is shorter than a 10 gallon? :icon_ques That contradicts the idea that plants need a minimum amount of light though. Maybe that minimum amount is 8 watts.  My 20high had 30 watts above it, and I had a harder time growing those plants then in the 6 gallon. (Now one of the ballast died on the 20, so it has 15 watts, yet I can still grow java moss, and barely, some anubias.)


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## epicfish (Sep 11, 2006)

Has anyone correlated these findings to Rex' thought on minimum lighting thresholds?

http://www.rexgrigg.com/mlt.html


I like to make things more confusing.


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## snafu (Oct 9, 2004)

Hoppy said:


> I don't see how this can be related to the substrate area, because the light at the point directly under the center of the bulb is the same for the same bulb no matter how big the substrate area is, and from my experience on a 10 gallon tank that light has to be much greater than for a bigger tank.


it's all about area. its about how many photons of a certain energy you can put over an *area* per unit *time*. in my simple world, i like to think of plants/leaves as energy conversion devices not unlike solar cells. leaves, like photovoltaics, have areas and conversion efficiencies. one converts photons into chemical energy and the other into electrical energy, but both require as input power over some area (or solar flux) incident on the collector. assuming the transfer function of the DC power into the bulbs to the energy directed into the tank were 100% efficient (which they are clearly not), you'd need twice the power to achieve the same flux for the 10 gal compared to the 40 gal over their respective areas. yes, directly under the bulb you may have a similar readings, but across the substrate area the average flux would be different for the two tanks given the same bulb. basically, the photons generated by the bulb are spread over a larger area. the attenuation given the variation in depth should be almost negligible provided there is somewhat low turbidity.


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## Hoppy (Dec 24, 2005)

If you are using a single tube light, with that light mounted across the center of the tank, the only effect substrate area has on light intensity is reflection from the front and back glass. If the area (front to back dimension) is large, there is far less of that reflection, and the further you get from the center of the tank the lower the intensity. That makes the intensity in a small tank of the same depth greater than in the large tank. Of course in reality, the larger the tank, the deeper it is, the wider it is, the longer it is, and the greater the front to back dimension. There are just too many variables to ever be able to calculate light intensity for any tank. The value of making an effort in a simplified form is that it allows you to do more accurate extrapolating when you have some data. Plus, it is just interesting to make the effort.


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## plantbrain (Dec 15, 2003)

Do you think that a small dinky, frankly....... crappy..........little 13W light, made for 10$ with ballast and fixture, vs a nice well designed 55 W light and ballast, much less "dead ends" are equal?

Have folks considered the bulb itself differences?
They are measurable BTW:thumbsup:

That should give you a good hint comparing.(not all of it, but some certainly)

Regards, 
Tom Barr


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## Ryzilla (Oct 29, 2005)

snafu said:


> it's all about area. its about how many photons of a certain energy you can put over an *area* per unit *time*. in my simple world, i like to think of plants/leaves as energy conversion devices not unlike solar cells. leaves, like photovoltaics, have areas and conversion efficiencies. one converts photons into chemical energy and the other into electrical energy, but both require as input power over some area (or solar flux) incident on the collector. assuming the transfer function of the DC power into the bulbs to the energy directed into the tank were 100% efficient (which they are clearly not), you'd need twice the power to achieve the same flux for the 10 gal compared to the 40 gal over their respective areas. yes, directly under the bulb you may have a similar readings, but across the substrate area the average flux would be different for the two tanks given the same bulb. basically, the photons generated by the bulb are spread over a larger area. the attenuation given the variation in depth should be almost negligible provided there is somewhat low turbidity.


In terms we can understand, you are saying that larger areas absorb more light. As the light source is emitted from high altitudes the larger areas with absorption properties will absorb more light even if the light source varies in intensity. If you reduce the altitude of the light source, and use a smaller absorption areas, the light source must emit at higher intensities to achieve the same effect of higher altitude light sources being emitted at larger areas of absorption.

If this is correct the amount of light needed over a certain tank footprint combined with its height determines the over head light requirement for plants.

I think what we need to do is eliminate the watts per gallon rule and implement the over head light intensity rule, that is what intensity of light is needed for the foot print and height of tank.. The watts per gallon rule assumes that the intensity of light remains the same at all altitudes in the tank, while the over head rule tells you what can be grown at the substrate.



plantbrain said:


> Do you think that a small dinky, frankly....... crappy..........little 13W light, made for 10$ with ballast and fixture, vs a nice well designed 55 W light and ballast, much less "dead ends" are equal?
> 
> Have folks considered the bulb itself differences?
> They are measurable BTW:thumbsup:
> ...


All things being equal but light intensity(Watts), altitude of light emission(height of light over tank from the substrate), and area of light absorption(foot print of tank)

I am no expert but for the sake of understanding, conversation, and experimentation I thought I would add my comments to this post.


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## Hoppy (Dec 24, 2005)

If you suspend a 24 inch long T5 light with its reflector above a table, with no tank there at all, you get a certain light intensity, in PAR units, on the table, and that intensity drops as you move away from the center of the fixture over the table. If you now put an empty 24 inch long glass tank under that fixture, on the table, the light intensity on the bottom of the tank will be greater than it was on the table, because some light reflects off the glass sides of the tank. If you replace that tank with one that has front to back dimension 4 times greater, the intensity on the bottom of the tank drops because the reflected light has to travel further to reach the bottom. Now, add water to the tank. Two things happen: the water very slightly reduces the intensity due to absorption, and, more significantly, the reflections from the glass are different, but I'm not clear how big an effect that is. For one thing the amount of light lost through the glass is different with water in the tank.

Separately, put the light just over the table again. Raise it and the intensity drops roughly proportional to the distance from bulb to table, but becoming closer to being proportional to the square of the distance as you move away from the center of the bulb. With the tank under the light all that changes from the above is the starting intensity.


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## SpeedEuphoria (Aug 5, 2008)

I agree that WPG is crude as do most. Reflectors and bulb type need to be accounted for which is a large variable.


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## snafu (Oct 9, 2004)

Ryzilla, not sure that's what i said or meant, but if it helps you understand that's great. i was just reiterating what Wasserpest said originally, which was a) that the substrate area per gallon is higher for smaller tanks and b) therefore you need to provide more power to achieve the same incident flux on a per gallon basis for the smaller tank. i'm not sure, how to say that more clearly.


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## Captain Hooked (Oct 13, 2008)

epicfish said:


> Has anyone correlated these findings to Rex' thought on minimum lighting thresholds?


Hi Epic',

I jokingly did (back in post 12), but I think Rex is right on the button about it. One caveat, though and it is in regards to lumens and how a lumen is a relative measurement to the _human eye_. Lightbulb companies made it hard to compare apples to apples. Rex's LUX and LSI mean very little in the way a _plant_ sees and reacts to light (as he very well knows), but in his defense- its the only way to measure light intensity (to date). I think it's time a new standard was developed as it relates to "true" light intensity, so that the WPG rule can be a laughable part of the hobby's past, and LUX can left to the lightbulb industry's "human" sector. 

To further illustrate how there is no real way to accurately quantify what a plant needs WRT the WPG "rule": Equal watts among CF, MH, HPS bulbs (et al) will all have different LUX. Add to that the variances of spectrum, depth, plant species, bulb manufacturing tolerances, falloff rates, etc- and the "true" light intensity will be all over the place. WPG is a worthless, Edisonian, antiquated way to measure anything.....except juice. Time for a new standard that uses mechanical photo receptors (which actuate a _plant_) as its benchmark.


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## Hoppy (Dec 24, 2005)

The single value of watts per gallon is that it is a known number. You buy any light or light fixture and the wattage is known. Measure the tank, and the gallons is known. So, you have a starting point for starting to think about what fixture or lights to buy. PAR is the much more relevant measure of light for an aquarium, but we don't know that for any light, fixture, bulb, or anything else. We need to use the PAR meters we bought at the bargain price to accumulate some data, such as PAR values at the substrate level for the fixtures we have, with various numbers of bulbs installed, for a specific size and shape aquarium. Add to that data some impressions about how well the plants do and how poorly the algae do, and we can start to be able to decide on lighting without even thinking about watts per gallon.

This is done to some extent now, in that people with good results with X fixture on a Y tank tell others, so it becomes accepted that X with Y is a good combination.


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## mistergreen (Dec 9, 2006)

have you considered maybe the amount of photons generated is exponentially or linearly related to wattage?

and Bar brought up a good point about ballast efficiency as well.


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## Hoppy (Dec 24, 2005)

It is entirely possible that people with 10 gallon tanks are on a very tight budget, or they would have bigger tanks, so they get an inexpensive light fixture. And, most light fixtures for that size tank are aimed at those without a lot of money to spend. So, it is possible that the lights used on that size tank are simply inefficient for all of the reasons he mentioned, and if the 10 gallon tank actually had a short T5 light of the same quality as is used on large tanks, it would require no more wattage than we would expect based on the large tanks. But, if I assumed all of that were the only factor involved I would have missed all of the fun of relearning some of my calculus skills, my geometry and trigonometry skills, and my reasoning skills. So, Tom gets his fun from measuring things and I get mine from calculating things. The result - we are both happy!! And, you guys get to read about our adventures.


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## lauraleellbp (Feb 3, 2008)

Hoppy said:


> It is entirely possible that people with 10 gallon tanks are on a very tight budget, or they would have bigger tanks, so they get an inexpensive light fixture. And, most light fixtures for that size tank are aimed at those without a lot of money to spend. So, it is possible that the lights used on that size tank are simply inefficient for all of the reasons he mentioned, and if the 10 gallon tank actually had a short T5 light of the same quality as is used on large tanks, it would require no more wattage than we would expect based on the large tanks. _*But, if I assumed all of that were the only factor involved I would have missed all of the fun of relearning some of my calculus skills, my geometry and trigonometry skills, and my reasoning skills. So, Tom gets his fun from measuring things and I get mine from calculating things. The result - we are both happy!! And, you guys get to read about our adventures*_.


:thumbsup::fish::smile:


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## plantbrain (Dec 15, 2003)

My calculating days are long gone
I do not trust myself.

I test, manipulate, collect and then do stat's, then if I am lucky....
I can make some conclusions.

19/20 times I louse something up or something is not right to make the statements I think might be true but am never quite really sure. But when the opportunity presents itself and the low picked fruit is there, why not?

Most of the issues I've figured out over the years in the hobby are really 3 types: low picked fruit, falsifying some hair brained claim/well thought out claim and practical common sense.

I left the PAR meter with Steve and George at Aqua Forest and went over with George a number of ways to use the meter. They should have fun. 

While calculations are fun, I've found folks comprehend things really well if you put it in their hand, show them in person directly and in plain simple down to earth terms.

I do, otherwise I sit and think a while
I'm pretty dense. 

Regards, 
Tom Barr


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## Basilisk (Mar 1, 2004)

Hoppy said:


> and if the 10 gallon tank actually had a short T5 light of the same quality as is used on large tanks, it would require no more wattage than we would expect based on the large tanks.


For what is worth, I have a 19" 16w T4, 6400K, which is bright enough, more than the NO 22" 14w T5. I get ballast, endcaps and bulb for US$10 (equivalent). I have yet to compare it to the HO 22" 24w.

As for the quality, it seems quite fine to me. Curious about a PAR measure.


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## Captain Hooked (Oct 13, 2008)

Hoppy said:


> The single value of watts per gallon is that it is a known number. You buy any light or light fixture and the wattage is known. Measure the tank, and the gallons is known. So, you have a starting point for starting to think about what fixture or lights to buy.


It might be helpful for newbies to know that the WPG "rule" is relative to so many factors as to make it quite meaningless. As I understand it, Watts are a measure of energy (a joule per second) and not in any way a measure of light output. Moreover, different bulb _types_ require differing wattages to produce equal luminosities. The only thing "known" (if you could call it that) about watts as it relates to this hobby is that a given light bulb _type_ (be it MH, CF, LED, et al) has a rough LUMEN PER WATT value rating. However, this is a very crude number because once you add the complexity of spectrum (among other things), it changes drastically. For example, the watts required by a CF bulb producing blue wavelengths will be different for yellow ones (if "equal luminosity" is to be maintained). This is only one of the many variables which make comparing watts to gallons a worthless endeavor. 

It's like giving 6 different people a ham sandwich and expecting them to run the same time in a 100 yard dash. On top of that- you're expecting them to run with the same _stride_. No way.


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## plantbrain (Dec 15, 2003)

Bulbs types alone will not tell you everything, so such comparisons, while well meaning, have other factors, the distance measured, the ballast types, the reflectors, the bulb ages, the region where the bulb is measured, (middle, ends etc) running the bulbs for a few min/hours first, ambient light etc.

Unless those are equal and mentioned, you can have very different data with the same bulb types.

Be careful about errors.
"Information" is not knowledge.
Data in and of itself is not that useful unless you have a good idea what you want to ask and know and then go about finding relevant data to help answer the questions.


Regards, 
Tom Barr


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## Captain Hooked (Oct 13, 2008)

plantbrain said:


> (snip)....such comparisons, while well meaning, have other factors, the distance measured, the ballast types, the reflectors, the bulb ages, the region where the bulb is measured, (middle, ends etc) running the bulbs for a few min/hours first, ambient light etc.
> 
> Unless those are equal and mentioned, you can have very different data with the same bulb types.



Exactly. To be scientific about it- you'd need a standard method for measuring, in addition to....


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## Hoppy (Dec 24, 2005)

Still, you have to have some method for determining how much of what light to use on the tank you have just obtained. Just saying it is impossible to know, and relying on a PAR meter means we have to guess at it, buy a fixture, install it, measure the intensity, then buy another fixture to get closer to the right amount, etc. 

I much prefer a crude way to guess at what is needed, combined with suspending the light above the tank so the intensity can be fine tuned by raising or lowering it. At best theorizing will give us an idea about how the light intensity varies with tank shape, so we have one more tool to use to fine tune our guess about the right fixture/bulbs to use.


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## plantbrain (Dec 15, 2003)

Thus if you get "crude enough", you end up back at 2 w/gal as a good starting standard, a range that's yet to have failed me.............

The solution needs to improve upon the existing problem with the crude rule of Watt/gal, and to date, I've not seen anything that does it that well and it becomes more and more complex. The PAR meter is simple, borrowing it and playing around is far more informative and addresses all the other issues.

While the trade off is the cost, pooled together and shared, since it's not something most will use daily, weekly etc, that can be overcome and tested.

Now you have something to work with and they are very easy to use, much simpler and takes less expertise than any NO3, KH, pH test etc. 
You have to be careful when your goal to improve something makes it become insignificant in your assumptions/trade offs.

You do not get far this way.

Regards, 
Tom Barr


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## Hoppy (Dec 24, 2005)

Tom, that's why I still like the "2 watts per gallon" rule, crude as it is. I always think of it as being 2 watts per gallon using AH Supply quality bulbs and reflectors, or 1.5 or fewer watts per gallon using T5 with individual reflectors for each bulb. Just a little more complicated, and it appears that those numbers may still be too high for comfort.

Of course, then we need to add, "but small tanks 10 gallon or smaller take more, and large tanks take less". The simple rule gets less simple the more we qualify it. But, it is still a good starting point, and it may prevent someone buying a 4 watt per gallon fixture for a 55 gallon tank.


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## Captain Hooked (Oct 13, 2008)

Using the 2 WPG rule for a 50G tank would give you 1,500 lumens of incandescent light, and 20,000 lumens of LED light. 

As a relative newbie to this hobby, I was frustrated while trying to decipher several things, lighting being chief among them. The problem was, I knew a thing 
or two about light bulbs beforehand, and was mystified by the WPG rule of thumb because no one was qualifying what _type_ of bulb the rule was _referring_ too.



To illustrate what I mean to the newbies, here is a basic comparison of various bulb types, and how many lumens they produce when given a single watt of juice.

Incandescent: Average household incandescent bulbs  
produce 15 lumens per watt. This is typical. The higher 
luminosity ones boast extremely high temperature 
filament burn and are used in specialty applications such 
as projectors. 

Halogen: The best ones can produce double the efficiency of your average incandescent at 30 lumens per watt.

Mercury Vapor: A good one can produce double the 
efficiency of a halogen at 63 lumens per watt.

Compact Fluorescent: Surprisingly, Compact Fluorescent bulbs are only slightly more efficient than Mercury Vapor bulbs at up to 80 lumens per watt.

Fluorescent (Tubular High Output): These are getting 
better all the time and good ones produce better than 100 lumens per watt. 

High (and Low) Pressure Sodium: Decent ones can easily double the efficiency of mercury vapor at greater than 120 lumens per watt. 

Metal Halide: Metal Halide bulbs share similar efficiencies to High/Low Pressure Sodium types. 

LED: Like the High Output Fluorescents, LED technology is rapidly changing. The latest CREE type LED's can produce better than 200 lumens per watt! This impressive level of efficiency is further enhanced by a staggering 80,000 hour lifespan (or more). It's just the beginning for high output LED's. Once manufacturing costs have stabilized, LEDs will become "the" bulb of the future in both residential and commercial applications.

From one extreme to the other, we see that LED's are 
capable of producing more than 13X the luminosity of an 
average incandescent. 

It would have been better (it seemed to me at the time) to discuss how many _lumens_ (PAR would be better)are required per gallon. This is similar to what Rex discusses, yet he illustrates square inches as being more accurate than gallons. I agree. I found it amazing how few manufacturers displayed the luminosity (LUX) of their bulbs. Go figure.... It had to do with spectrum.

When you are talking about a specific spectrum (wavelength), those efficiency ratings I listed go out the window. This is because no benchmark exists for measuring and reporting that data. Whether this is due to the fact we are using an old standard, or a marketing scheme of the lighting industry- I don't know. Suffice to say that it makes it very hard to compare light bulbs in the ways it matters most to us. 

To further complicate matters, we are discussing 
"lumens", which is flawed to begin with because lumens are a measure of how the human eye sees light. Humans are especially sensitive to the green and yellow wavelengths, and these are the ones most plants make little use of. 

In the end, I realized it didn't matter much because there 
is a huge margin for error. I came to this conclusion:

The WPG "rule" does not work because it is remotely accurate or scientific. What "works" (kinda/sorta) is sticking just about any bulb above your plants (near that rule) and going from there- so long as that bulb is of the appropriate spectrum and is not terribly overkill. Is this because most lights are good? I don't think so. I think it is because plants are a living organism, and they make the appropriate adjustments to compensate for our crude lighting. The reason the WPG rule has any snowballs chance of working is testament to a plant's adaptability.


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## epicfish (Sep 11, 2006)

From what I gathered when I first started out, the WPG was based upon straight, twin-tube power compact lights. The approximate WPG ranges have yet to fail me while using PC lighting on tanks with volumes falling within the range of the WPG "rule".

Spiral CFL would have ~40-50% the efficiency of these power compacts, and as Hoppy said, I considered T5 to be 130-150% as efficient as these power compacts.


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## Captain Hooked (Oct 13, 2008)

epicfish said:


> From what I gathered when I first started out, the WPG was based upon straight, twin-tube power compact lights.


 
I wish I knew you then! I (obviously) never knew there was such a standard.


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## Hilde (May 19, 2008)

Hoppy said:


> Figuring all of that out for various tank sizes gives the following table:
> 
> 
> 
> ...


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## Hilde (May 19, 2008)

Captain Hooked said:


> The WPG "rule" does not work because it is remotely accurate or scientific.



Oh, and guess what! The wpg rule is based on th output of T12 bulbs. 



Captain Hooked said:


> I think it is because plants are a living organism, and they make the appropriate adjustments to compensate for our crude lighting.


Here is a link that proves your theory. 
http://www.barrreport.com/general-p...ting-bulbs-plants-2.html?highlight=wavelength

Also I have heard some have great plant growth with T8 daylight bulbs. 
I use the T5s for I good deal on a fixture that uses T5 bulbs. I have a dual bulb with 10,000k and 6500k. I think the 6500k is better for view and plant growth.

Here are some examples of different bulbs.
*http://www.theplantedtank.co.uk/lighting.htm*


So what lights did you end up using and what size is your tank?
*http://www.theplantedtank.co.uk/lighting.htm*
*
*
*http://www.theplantedtank.co.uk/lighting.htm*


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## Hilde (May 19, 2008)

Hoppy said:


> Well is time to eat some crow - I do this often enough to have developed a taste for it.


And how do you flavor that crow, ketchup? Oh this thread has been so humorous. My head is spinning now.


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## Homer_Simpson (May 10, 2007)

Obervation IME and IMHO may not be the most scientific way of determining how much light your tank needs but it appears the most telling. A lot depends on your goals. Are you planning to go full throttle with pressurized c02 and demanding plants. If so, you may find that you need to follow the scientific defintion of proper lighting for a small tank. If you are planning to go with low light plants, then the science may not apply. I have 2 low tech 5 gallon tanks with 13 watt compact fluorescent bulbs, no c02, Excel dosing daily. If I go switch to a 14 watt or higher bulb I notice the materialization of green spot algae on the tank walls within days, even with 8 hours photoperiod. If I go down to 13 watts again, the algae eventually recedes and disappears. When you think about it, scientifically 14 watts would not be considered excessive light for a 5 gallon and probably not even sufficient for low light. We have to be careful not to mislead newbies and set them up for failure by making them think that they cannot succeed unless they meet the scientific definition of what constitutes sufficient lighting for their tanks.


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## talontsiawd (Oct 19, 2008)

Very interesting. I would have thought that height would be a "limiting" factor but it makes sence that a few inches may not be nearly as important as how "wide" the light is (which is evident on my 10 gallon with spiral CF bulbs). How many lumens a plant needs being non linear to tank size makes sense as well. I'm not even sure this is what was meant, i'm still totally confused about the whole thing but good thread.


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## Shad0w (Sep 26, 2007)

All are so confusing . what I normally do was just put as much as light set that can fit on top of the tank. My 3ftx1.5ftx1.5ft tank have 6x39W T5HO on top. The down side is I pay more electricity bill, dose more fertilizer, inject more CO2. :red_mouth

It may not be the best way but save my headache


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## Hoppy (Dec 24, 2005)

Time has been marching along since I started this thread over a year ago. Since then, like most of us, I have learned a few things. That makes me believe that the conclusions in this thread are not accurate at all.

The mistake I made, and which most are still making, is to accept that the wattage of a bulb has something to do with how much light that bulb produces in lighting an aquarium. It doesn't. Any type of bulb, from T12 to T5HO, produces about the same light intensity directly below it, whatever the wattage of the bulb is. In other words all T12 bulbs produce about the same intensity directly below them - the higher wattage bulbs are just longer, so they produce that intensity over a longer aquarium. Place a 40 watt T12 bulb or a 20 watt T12 bulb over a ten gallon tank and you get the same intensity of lighting.

Small tanks don't require more light than large tanks, and very large tanks don't require less light than medium tanks. The higher the tank the more light it needs to get adequate intensity at the substrate. The greater the front to back depth the more bulbs are needed at the top of the tank to get good uniformity of lighting throughout the tank. It is that simple.


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## Church (Sep 14, 2004)

Hoppy said:


> Any type of bulb, from T12 to T5HO, produces about the same light intensity directly below it, whatever the wattage of the bulb is ... It is that simple.


You can't be serious, right? :icon_conf

I've never even seen this thread before now, but I'm about to read it...


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## nismo tetra (Oct 11, 2009)

I am going through this same "smaller needs more" frustration with my 5.5 gallon. I have 2x9w 6,700k CF over it. Problem is the fixture is not long enough(6 inch bulbs) so the edges of the tank are significantly darker. Now last night in my junk pile I found a 16 inch single tube fixture that uses a 15 inch T8 14w bulb. It lites the tank much much better, but I'm worried if it is intense enough to grow my plants. I need to get a new bulb, which they are somewhat hard to find. But I did find a GE 9,325k 14w bulb that I think I may try out. http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3733+8066+3810&pcatid=3810

Another question on that, it has a ballast in line with plug. It's an oceanic fixture (could be 15+ years old lol) and on the inside of the light fixture it says 22w. Does that mean I can use a 22w bulb if I can find one?

All I have in there is some elocharis vivipara, elocharis parvula, and some mayaca fluviatilis. Was wanting to maybe try some HC on the top of the tall rocks (few inches under the light fixture). Not sure if it will get the job done though.

Do you guys think that single bulb would be sufficient for the plants?


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## Hoppy (Dec 24, 2005)

Church said:


> You can't be serious, right? :icon_conf
> 
> I've never even seen this thread before now, but I'm about to read it...


Yes, I'm serious. Notice that the wattage of a fluorescent bulb is approximately proportional to its length. A 48 inch T5HO bulb uses 54 watts, a 24 inch bulb uses 24 watts, a 60 inch bulb uses 80 watts, or 1.1 watts per inch, 1 watt per inch and 1.1 watts per inch. As the bulbs get shorter than that, the non-tube portion of the bulb becomes a much larger percentage of the total length, so the relationship breaks down. In T12, a 48 inch is 40 watts, and a 24 inch is 20 watts. The bulb length just determines how much power it takes to drive the bulb, not the brightness of it.


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## lauraleellbp (Feb 3, 2008)

Now I understand what you're saying, Hoppy. At first I thought you were saying the light intensity would be the same under a CF vs T5 vs T5HO bulb as long as they were all the same length, and that wasn't making sense to me.


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## Church (Sep 14, 2004)

Well then it would seem that running 24" fixtures over a 10g might be the best option to have choices with, in terms of whether to go low-light or high light, or anywhere inbetween. I mean, who cares if the fixture hangs over the edges of the tank by 2" on each side, right?

Hoppy, you said yourself that the relationship breaks down as the bulbs get smaller, so maybe that is directly related to the need for larger wattage on smaller tanks? That's why I'm saying maybe using longer bulbs might open up better possibilities.

Just thinking out loud...


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## Hilde (May 19, 2008)

Reread this thread and it still makes my head spin. I am planning to downgrade when I move to 10 gallon tank. Going to use T8 20 watt 24 long or PC 65 watt 21long. Which would work best without injecting Co2?


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## Hilde (May 19, 2008)

bump!! already stated.


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## Wasserpest (Jun 12, 2003)

Why not start a new thread for your specific tank and issues? I don't see how they apply to this discussion.


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## Hilde (May 19, 2008)

Church suggested it is better to have bulb 24 in bulb for a 10 gallon. Hoppy was talking about wattage needing to be higher for a 10 gallon. Thus I am confused as to which 1 gives the best light, T8 24 in or PC 65 65 watts 20in, for a 10 gallon tank.


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## Hoppy (Dec 24, 2005)

Don't think wattage, think type of bulb. T5HO is best for high tanks, T5NO for not quite so high tanks, PC for still lower tanks, and T8/T12 for low tanks. Then match the bulb length to the tank length. Finally, if the tank front to back dimension is comparable to its height, use more than one bulb to cover the whole substrate. Like most generalizations, this is just a .....generalization. And, for low light tanks, use lower efficiency types of light. (Another generalization.) And, if you raise the light higher above the tank that's the same as if the tank were a higher tank.


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## Hilde (May 19, 2008)

Hoppy said:


> Don't think wattage, think type of bulb.


Oh!!:icon_idea Now I am not so confused. Thank you. I shall save this info.


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## Hilde (May 19, 2008)

Hoppy said:


> T5HO is best for high tanks.


What is considered high?


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## lauraleellbp (Feb 3, 2008)

Hilde said:


> What is considered high?


24"+ usually


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## Hilde (May 19, 2008)

lauraleellbp said:


> 24"+ usually


Thanks!


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