# How to determine correct lighting (Watt per gallon) for the Small tank ?



## ppaskova (Apr 27, 2010)

Hi

I have a 10Gl fish tank and I'd like to know how to calculate Watt per Gallon for it. Is it regular calculation (e. g. if I have 2 X15 fluorescent scuin light bulbs) in it it considered 3WPG or as it considered a small tank it's a different calculation ? And can I put there regular spiral type scru in light bulbs or the light will not be sufficient enough for plants in the whole tank (specially in the corners) to grow ? All my plants are low light plants and I'm planning to replace my lights with Philips scu in light bulbs (2X9watt) to decrease algae grow ? Any help is appreciated

Thanks


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## Darkblade48 (Jan 28, 2008)

WPG is a very rough guideline that cannot really be used anymore due to the advent of so many new types of bulbs that are more efficient, etc.

If you have 2 x 15 watt CFL bulbs, then strictly speaking, you have 3 WPG over a 10 gallon tank. However, CFL bulbs lose quite a bit of light to restrike, and in addition, as you already know, the WPG guideline cannot really be used for aquariums smaller than 10 gallons anyway.

If you put in two CFL bulbs for a 10 gallon, it should be enough light for most plants.

If you decrease the light, it should still be OK.


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## waj8 (Jun 30, 2010)

The proper way to determine light levels is to take into account fixture efficiency and lamp efficiency. You need about 80 lumens per gallon for a low light tank and at least 320 lumens per gallon for a high light tank. Watts per gallon is such a rough guide because it does not take efficiency into account.

One way light output can be calculated is to find out the mean lumen output of your lamps if they were 841 lamps. 841 is the spectrum that is normally used for comparing fixture efficiencies. You then must determine the ballast factor. You must also derate the fixture if it does not have first class reflectors. For example, if you have a 15 watt cfl lamp it may produce 900 lumens with an 841 phosphor. The ballast is built in so you don't have to account for the ballast factor. The fixture efficiency is going to be pretty bad because of the shape of the lamp tube and the impossibility of making a specular reflector. I would derate the ficture by at least .8

Anywho, 900 (841) lumens x 1 lamp x 1.0 B.F. x .75 fixture efficiency / 10 gallons = 70 lumens per gallon. That is not likely to grow much.

If you have a 20 gallon tank with a 55 watt PC and good reflectors, lumen output would be 4800 lumens x (maybe) .95 B.F. x 1 lamp X 1.0 fixture eff. / 20 = 228 lumens per gallon. This tank would grow most plants. It would be about a 3 out of 5 on a scale of 1 to 5 and would allow you to grow nearly anything.

With some research you can find out the values you need to do the calculation but it is not easy and that's why people tend to go with the watts per gallon rule. 

If you have a fixture in mind I would be happy to calculate the efficiency and output for you. Don't forget that you must use the same 841 lamp spectrum to make meaningful comparisons regardless of what lamp you will actually be using. BTW I just pulled those examples out of my head so they are examples only and may not be exactly accurate.


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