# What is your watts per square foot?



## medicineman (Sep 28, 2005)

260 gallon
bottom area around 13.5 sq ft
450W Metal halide + 160W T8 (alternate full blast)
So it is around 11.9W per sq ft - 45.2W per sq ft
High light plants

20 gallon
bottom area around 3.1 sq ft
80W T8
Makes around 25.8W per sq ft
Medium light plants

Remember in high light tanks it is not needed to light all light potential all at once for the whole photoperiod. Instead it is enough to do 60-70% full blast and the rest of 9-10 hours with less than 1/2 the total light potential.


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## Rex Grigg (Dec 10, 2002)

55 gallon tank, 220 watts. Surface area approximately 4 square feet 50-55 watts per square foot.


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## Canoe2Can (Oct 31, 2004)

I used to have an odd shaped 45 gallon that was 3.125 sq. ft. with 130 Watts of PCs. 41.6 Watts per sq. foot.

Now I have a 120, 8 sq. ft., with 216 Watts of T-5s. However, I have them staggered such that there is 8 hours per day with 108 Watts and 3 with the full 216. 13.5 or 27 WPG. 

I agree with you that this is a better way to measure light over a tank. More predictable than Watts per gallon. As a reference point, an article on planted tanks I read some time ago recommended 25-50 Watts/sq. foot for medium to high light plants. But I don't remember if it made any recommendation for low light.


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## attack11 (May 5, 2006)

150w above 20"x18" so .. >75w?


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## SammyP (Apr 26, 2006)

my 10 gallon is 10x20 or 1.3 sqft with 76 watts that makes ~55.4 watts/sqft

my lights are setup similar to canoe2can. 6 hours of 27.7 watt/sqft and 3 hours of 55.4 watt/sqft


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## BlueRam (Sep 21, 2004)

Tank 1: 60 gal with 48X16" footprint and 128W T8 or *24W/sft* (medium light, medium tech)
Sump: 20 gal with 24X12" footprint and 55W PC or *28W/sft*

Tank 2: 20Long with 30X12" footprint and 65W PC or *26W/sft* (medium light, lower tech (no CO2, some hair algae))

Tank 3: 75 gal with 48X18" footprint and 64W T8 or *11W/sft* (low light, low tech)


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## matthewburk (Sep 22, 2005)

Thanks for all the replies. Seems like some of people have VERY high light imo. This is good info.


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## matthewburk (Sep 22, 2005)

BlueRam said:


> Tank 1: 60 gal with 48X16" footprint and 128W T8 or *24W/sft* (medium light, medium tech)
> Sump: 20 gal with 24X12" footprint and 55W PC or *28W/sft*
> 
> Tank 2: 20Long with 30X12" footprint and 65W PC or *26W/sft* (medium light, lower tech (no CO2, some hair algae))
> ...


So I'm interested in the lighting on your 75 gallon. That seems very low and what I want to do I think. What kind of plants do you have in that light Anubias and crptys?


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## matthewburk (Sep 22, 2005)

Canoe2Can said:


> I used to have an odd shaped 45 gallon that was 3.125 sq. ft. with 130 Watts of PCs. 41.6 Watts per sq. foot.
> 
> Now I have a 120, 8 sq. ft., with 216 Watts of T-5s. However, I have them staggered such that there is 8 hours per day with 108 Watts and 3 with the full 216. 13.5 or 27 WPG.
> 
> I agree with you that this is a better way to measure light over a tank. More predictable than Watts per gallon. As a reference point, an article on planted tanks I read some time ago recommended 25-50 Watts/sq. foot for medium to high light plants. But I don't remember if it made any recommendation for low light.


Oh I need to find that article, thanks for letting me know


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## Rex Grigg (Dec 10, 2002)

This method of measuring light is as flawed as the WPG rule of thumb.

Take three tanks.

30 gallon
38 gallon
45 gallon

They all have the same footprint. If you had 25 watts per square foot on each of these tanks you would have ~75 watts over each tank. But since you have depths of 16", 20", 24" you will have a loss of light intensity. Plants that grow well in the 30 gallon tank may well suffer stem rot in the 45 gallon. While depth of the tank is not a huge factor in most aquariums it becomes more significant at lower light levels.


There is also a more extreme example of this. There is a tank with the same foot print of a 10 gallon tank that is 24" deep.


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## BlueRam (Sep 21, 2004)

Bright for night  



matthewburk said:


> Seems like some of people have VERY high light imo.


In this tank hygro and bolbitus does well. This is a new tank (to me) so I have not done much yet. Details to be posted in a picture album at some point.



matthewburk said:


> So I'm interested in the lighting on your 75 gallon. That seems very low and what I want to do I think. What kind of plants do you have in that light Anubias and crptys?


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## matthewburk (Sep 22, 2005)

Rex Grigg said:


> This method of measuring light is as flawed as the WPG rule of thumb.
> 
> Take three tanks.
> 
> ...


I agree that it is as flawed as the WPG rule. But it is also a more intuitive rule, to me at least.


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## _NE_ (Jun 19, 2006)

For about a year ago i found a web site which calculated a value mostly dependent of the foot print of the tank but also took the height of it in consideration.
The site also had references of low, mid and high light tanks.
I have been searching a lot for this since but not been able to find it, anyone knows what I'm talking about?


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## _NE_ (Jun 19, 2006)

It was not like i remebered it, but i found it:

http://fins.actwin.com/mirror/plant-lighting.html


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## daFrimpster (Mar 7, 2005)

This is no better than WPG. Hey let's try cubic inches per watt. my 55 is approx 12096 cu inches. My lighting is 128watts. I have 94.5 CIW. sweet!!
But alas this too is flawed. Ohh the frustration!!


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## BlueRam (Sep 21, 2004)

I support people thinking about problems in new ways and trying to find a better answer. 

I think it is the relance on "Watt" that is flawed. If people were measureing intensity (Scolley shows a method with a CD) problems with wattage, reflectors, depth etc would diminish. We would still have the "lumens as measurement of anything" debate though.



daFrimpster said:


> This is no better than WPG. Hey let's try cubic inches per watt. my 55 is approx 12096 cu inches. My lighting is 128watts. I have 94.5 CIW. sweet!!
> But alas this too is flawed. Ohh the frustration!!


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## matthewburk (Sep 22, 2005)

daFrimpster said:


> This is no better than WPG. Hey let's try cubic inches per watt. my 55 is approx 12096 cu inches. My lighting is 128watts. I have 94.5 CIW. sweet!!
> But alas this too is flawed. Ohh the frustration!!


Calm down genius. Everyone knows this, and it was already pointed out in this thread for those who didn't, but by a much less annoying person.

What plants *probably care about is the intensity of the light and how long.

Light intensity is generally measured by the amount of light per square unit.

Therefore expressing watt per square unit is more intuitive to me becuase it is more closely related to how intensity is traditionally expressed.

Of course we use watts instead of actuall intensity becuase not many people have the means to measure the actuall intensity. 

Taking into account distance of the light source from the plants would be the next logical step in making a more useful rule of thumb, since intensity changes decreases with increased distance.



*I don't really know what plants care about, just an educated guess.


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## snafu (Oct 9, 2004)

Rex Grigg said:


> This method of measuring light is as flawed as the WPG rule of thumb.
> 
> Take three tanks.
> 
> ...


are you suggesting there is appreciable light attenuation in 4" and 8" of clear water. if you assume your water is like the green water near duluth in lake superior, which has a vertical extinction coefficient of 0.3 m-1, i suspect the difference in intensity would be less than 4% for the worst case. i would appreciate if someone could do the quick calc using the formula: 

i(d) = i(0)* exp(-k*d), 

where 
d is depth in meters
k is the attentuation coefficient
i(0) is the intensity at depth 0 meters (at surface)
i(d) is the intensity at depth d meters

(ref. http://waterontheweb.org/under/lakeecology/04_light.html)
-snafu


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## jgc (Jul 6, 2005)

55 watts/ft.


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## attack11 (May 5, 2006)

lol @ snafu. i didn't wanna be the one to point that out 

you need like 10' of clear water depth before the light is really affected.


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## Rex Grigg (Dec 10, 2002)

You need 10' of light to affect full SUNLIGHT. Our tanks are a different story.

I suppose you jokers missed this sentence:



> While depth of the tank is not a huge factor in most aquariums it becomes more significant at lower light levels.


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## attack11 (May 5, 2006)

well, that's stating the obvious. 4" of depth of clean/clear water won't affect your plants.


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## Rex Grigg (Dec 10, 2002)

When compared to sunlight our tanks are low light. Also water scatters light. So the intensity is always highest right at the surface. Once the light starts traveling through the water it gets scattered. And that means that the light will start to exit the tank.

Another example is a 75 gallon vs a 90 gallon tank. Same foot print but the 90 gallon is 4" deeper.

Now imagine you have JUST ENOUGH light to grow a ground cover in the 75. Put the same lighting over the 90 and the ground cover will most likely still grow. But it will either grow slower or grow taller trying to reach the light.

I know I have stated several times in the past that tank depth makes almost no difference. But we are talking about extremely low levels of light here. That's what makes the difference.

And let us not forget that most aquarium water "appears" to be clear. But it's not. It has higher levels of DOC, TDS, and things in it than most any water commonly found in nature.


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## attack11 (May 5, 2006)

i'm pretty sure everyone knows that, we're on this forum trying to grow small ecosystems 

what you're not factoring into your 75 vs 90 example is the different of water chemistry and all the other variables. if you can grow at 75gl you should be able to grow at 90gl with the same light if you take all the differences into account, unless it's a drastically different shape and the area doesn't get similiar coverage. i've put my 80w pc fixture on my 15gl after i moved to mh on my 47gl and the plants responded the same, the water was the same, the substrate was similiar but obviously the bacteria setup was different. the height difference between the two tanks is 16".

here is a setup pic of the small tank with some bga on the lonely rotala. i'm gonna restart this tank i think, i never did anything with it.


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## relaxing (May 10, 2006)

34.67 W/ft2


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## observant_imp (Jun 30, 2004)

For rectangular tanks I could see using a set of tables. 

On one side you use depth ranges (12-15, 15-18, 18-24), on the other side you could have either lighting type (fluorescent, pc, etc.) or lighting intensity (high, medium, low). 

In the cell you put the watts per linear (orsquare) foot. 

If you put lighting type on the side, you have a table for each lighting intensity. If you put lighting intensity on the side, you have a table for each type of lighting.

I use PC lighting on most of my tanks, so 32.5 watts per linear foot (or 65 if I run it bright).


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## BlueRam (Sep 21, 2004)

I would like to see a modern version of this plot from thekrib:
http://www.thekrib.com/Plants/Tech/Lighting/

Kinda the same ideas in metric units...



observant_imp said:


> For rectangular tanks I could see using a set of tables.
> 
> On one side you use depth ranges (12-15, 15-18, 18-24), on the other side you could have either lighting type (fluorescent, pc, etc.) or lighting intensity (high, medium, low).


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## vidiots (Jun 17, 2006)

I think, you are repeating the same flaw as the watts per gallon rule trying to use any sort of cubic measure. They are both volume units. Infact the official definition of the U.S. gallon is 231 cubic inches. It looks different but your basically just converting from one volume unit to another. It's like converting Liters to Gallons. By the way the definition of the Liter is 1000 cubic centimeters.

I think you are on the right track with the intensity per square foot, with simplified corrections for lighting type, depth, bulb age, and spectrum. They don't have to be precise corrections, just ball park figures.

To answer the origional post I have a 180Gal tank that is 72"x24"x24" with 3 175w Metal Halide Bulbs and 4 65w PC flourecents for a total of 785w or 65w/ft^2 (4.36wpg) I consider this very high light. Probably considered overkill by most, but hey the fishies get a nice tan, and I can extend daylight hours for half the neighborhood.


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## BlueRam (Sep 21, 2004)

On WPG I gave my reasons why Watts are not the best above, vidiots gives reasons why gallons are not ideal so now we just need to hear from the "per" camp  

Not to outgeek here but in pure metric units a liter would be a cubic meter. Right up there with food calorie = 1 kcal. :icon_roll 



vidiots said:


> I think, you are repeating the same flaw as the watts per gallon rule trying to use any sort of cubic measure. They are both volume units. Infact the official definition of the U.S. gallon is 231 cubic inches. It looks different but your basically just converting from one volume unit to another. It's like converting Liters to Gallons. By the way the definition of the Liter is 1000 cubic centimeters.


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## attack11 (May 5, 2006)

1 cubic meter is 1000 cubic centimeters 

getting way off topic, the imperial/metric battle is really a pain when comes to hose. it seems the lfs stock pond hose which is metric, but no hardware store does! it's all imperial. such a headache.


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## BlueRam (Sep 21, 2004)

So one demerit for me as I was looking for "swirly eyes" and ended up with rolling eyes :icon_conf

If 1 m = 100 cm.
Then 1M^3 = (100)^3 = 1,000,000 cm^3 or 1000L.
*Primal Geek Scream!* 




attack11 said:


> 1 cubic meter is 1000 cubic centimeters


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## attack11 (May 5, 2006)

ah. the typo. meant 100cm


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## Canoe2Can (Oct 31, 2004)

vidiots said:


> I think, you are repeating the same flaw as the watts per gallon rule trying to use any sort of cubic measure. They are both volume units.


Since when did square feet become a measurement of volume?

I think that this method is still unreliable, but less so than Watts per gallon. If you start with Watts per square foot, then factor in another variable for the height of the tank (especially if it's out of the ordinary), you'll come up with something close. Especially when we remember that all flourescent lights are at least in the same neighborhood in terms of lumens per Watt. Provided everyone has good reflectors, it can be a good approximation.

But I'd have to say that the BEST way of determining how much light you need is to ask other people who have the same tank what has worked for them.


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## daFrimpster (Mar 7, 2005)

matthewburk said:


> Calm down genius.


aw shucks! thanks for noticing!


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## g8wayg8r (Dec 24, 2003)

I have 44W/sq ft and my tank is 18" deep. Growth of high-light plants is marginal near the substrate and down right amazing a few inches from the surface. Depth does have big effect. From 0 to 18 inches, the effect is huge. Since most tanks (I presume) are in the range of 12 to 24 inches deep, I do believe selecting high intensity bulbs is critical but the trouble is that the intensity of compact flourescent bulbs is similar, regardless of the length of the tube, i.e., the wattage.


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