# Light Height above Tank



## Hoppy (Dec 24, 2005)

I have an 8 gallon nano cube tank set up as a desk aquarium, with two screw-in CFL 10 watt bulbs lighting it. I used a PAR meter to determine that I have 50 micromols of PAR at the substrate, which is low medium light. I'm getting some algae growth on the glass.









Studying this I can tell that my problem is that the two bulbs are barely above the water level in the tank, so they are close to the glass, causing very high light at that location. I think this is probably a clue about how to avoid GDA and GSA, or at least how to greatly reduce them. The solution is to suspend the lights high above the tank, to reduce the maximum light intensity in the tank. Some sketching and rough measuring demonstrates this:









As long as I have had aquariums the conventional, standard, normal way to light a tank has been to sit a light fixture on the top of the tank. This gets the most light into the tank per watt of power used to power the lights.

For a few years now people have been hanging their light fixtures above the top of the tank, especially with open top, rimless tanks. One major advantage of this is the ease of adjusting the light intensity by raising and lowering the height of the fixture above the tank. But, now I think the most important advantage is the reduction of high light on the upper parts of the tank, on the high plants, and on hardscape, like filter tubes, electric wires, etc. located at the top of the tank. This may be the major reason why those who use this lighting method, and who have low to medium light at the substrate level, have far fewer algae problems - they just don't have any high light areas of their tanks.

I believe this should become the conventional, standard, normal way to light all of our tanks.


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## jcardona1 (Jun 27, 2008)

Great thread Hoppy. And I can attest to that with the PAR meter I borrowed from you guys. On my 57g tank with two T5HOs suspended several inches above the tank, I noticed big differences in PAR at the same level, but just moving the meter to different locations. 

Moving the meter close the glass showed a big increase in PAR, about 5-10 micromols.


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## barbarossa4122 (Jan 16, 2010)

Great work Hoppy. I wish I have a PAR meter. I would like to rent one but, I do not know who'll rent it in NYC area.


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## offpath (Jan 18, 2009)

Agreed, Hoppy. I came to many of the same conclusions when I was modelling the lighting for my 210 gallon tank that we discussed in your PAR thread. I wrote a program to look at the estimated PAR at various points in the tank, just using your charts and the square of the distance from each individual bulb. Even with the lights about a foot above the tank, the light intensity at the top of the tank was enormous compared to the substrate!


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## efface (Aug 27, 2010)

Looking at your diagram and you mentioning you would have to double the output to get the same amount of light at the substrate......wouldn't that just double the light hitting the glass and undermine the fact you raised the light up to prevent/slow algae growth?


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## Hoppy (Dec 24, 2005)

Surely there is an aquatic plant club in NYC. Here is a list of clubs around the world, http://www.aquatic-gardeners.org/links.html#local and another resource, http://www.plantedtank.net/forums/new-york/

Some aquatic plant clubs bought their own PAR meters for member's use, so that's the best place to start looking.


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## Hoppy (Dec 24, 2005)

efface said:


> Looking at your diagram and you mentioning you would have to double the output to get the same amount of light at the substrate......wouldn't that just double the light hitting the glass and undermine the fact you raised the light up to prevent/slow algae growth?


Assuming the intensity was 50 micromols at the substrate in both cases, the intensity near the top would be far less with the light raised a height equal to the tank height above the top of the tank, than with the light sitting on top of the tank. But, you would need 4 times the number of bulbs or emitted light from the bulb, for the high hanging light - twice as far from the substrate, so 4 times as much emitted light needed. If you used Solatube skylights to light the tank, the intensity should be the same all over the tank.

This is more complicated than this, because some light reflects off the glass sides to increase the light near the glass, but it is a good approximation. That reflected light would tend to make the intensity at the substrate more nearly uniform, especially with a high hanging light.


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## barbarossa4122 (Jan 16, 2010)

Hoppy said:


> Surely there is an aquatic plant club in NYC. Here is a list of clubs around the world, http://www.aquatic-gardeners.org/links.html#local and another resource, http://www.plantedtank.net/forums/new-york/
> 
> Some aquatic plant clubs bought their own PAR meters for member's use, so that's the best place to start looking.


Thanks for the links Hoppy.

I asked on another forum if I can rent one and I am waiting for replies. I am willing to send a refundable deposit for the cost of the meter.


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## offpath (Jan 18, 2009)

efface said:


> Looking at your diagram and you mentioning you would have to double the output to get the same amount of light at the substrate......wouldn't that just double the light hitting the glass and undermine the fact you raised the light up to prevent/slow algae growth?


The key point here is that light intensity falls off as the square of the distance, not linearly. So, the further your lights are from the top of the tank, the more even the intensity is at all points in the tank, even if you have to use more light in total.

To take an extreme example, if you were to use sunlight to light your tank, the sun is so far away that the intensity of the light would be identical at the top and bottom of your tank.


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## outcast (Jul 4, 2007)

great thread, i dont know why this and the other par thread you did arent stickied here. they really bring ease explaining to people about lighting


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## odie (Dec 29, 2009)

Hoppy,
Again thanx for all of your wisdom and hard work in making things easier for many of us!!

Now I know so far nobody has mentioned their own setups or asked for advice....so I will break the seal!!

In setting up my 125 gallon 6 ft tank (plants on order). I have 2x39watt 36 inch T-5 HO per side. 78 watts per side or 156 watts over the length of the aquarium. the fixture is a Catalina that was a 4 or bulb fixture but with only 2 bulbs to help spread the light. It also doesnt have the best reflectors in it due to this.

Aquarium is 22 inches and I am using the "legs" that came with the fixture. I am guessing it raises it 3-4 inches. 

My question(s) is- Should I raise this fixture more? Will this need co2 to balance light out? I will have it on a split light cycle. So 8 hours with a one hour break after 4 hours.

Once again thanx Hoppy and everyone else!!


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## Hoppy (Dec 24, 2005)

odie said:


> Hoppy,
> Again thanx for all of your wisdom and hard work in making things easier for many of us!!
> 
> Now I know so far nobody has mentioned their own setups or asked for advice....so I will break the seal!!
> ...


I think you will most likely have medium light with that setup. CO2 will be very desirable with that much light, to avoid algae problems. It isn't that the CO2 balances out the light, it just gives the plants the available carbon to grow at the rate the light drives them to grow. This results in healthy plants, which is the best way to avoid algae. If you prefer not to use pressurized CO2, you could raise the lights another 4-6 inches to get low light. An advantage of having the lights hanging instead of sitting on the feet, is that you can easily adjust their height to get whatever light intensity you want.


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## BayBoy1205 (Jan 25, 2006)

Hoppy, Just wondering what PAR meter you use and where might one be bought?


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## tom855 (Jan 31, 2006)

I just took the plunge and bought one yesterday from Aquarium Specialty. It was the Apogee MQ-200. $335 delivered.

.


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## Hilde (May 19, 2008)

offpath said:


> Agreed, Hoppy. I came to many of the same conclusions when I was modelling the lighting for my 210 gallon tank that we discussed in your PAR thread. I wrote a program to look at the estimated PAR at various points in the tank, just using your charts and the square of the distance from each individual bulb. Even with the lights about a foot above the tank, the light intensity at the top of the tank was enormous compared to the substrate!


I realize this is an old tread. I need to understand this better though if I am to use a Element Green LED strip with 16x 6,500K 3w LED over my 20G high tank. It is 15in high.Also wondering if I raise the light 6in above the tank can I use an 18in strip which has 10x 6,500K 3w LEDs?

Got a link to the program? Can it be used with an excel sheet?


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