# is there a rule for lumens per gallon?



## Dendro Dynasty (Jul 11, 2013)

im wondering how many lumens is too much? I have a 60 gal with 4000 lumens the plants seem to be doing great, but really my question is for smaller tanks. is there a basic rule of thumb for selecting the right size lights for diff size tanks?


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## Ray (May 12, 2009)

The volume of the tank is irrelevant; it's the distance from the light source to the plants that matters.

Light intensity falls away as the inverse square of the distance. If plants in one tank are one foot from the lamp, and are getting "_X_" foot candles, then ones in a bigger tank, that puts them 2 feet from it are getting "1/4_X_", so if they need the same amount of light, you need 4 bulbs.


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## Dendro Dave (Aug 2, 2005)

This is a guideline not a rule, but I find for T-5/t-8 fluorescent, or CFL fluorescent that 2-3.5 watts per gallon (More if the tank is taller then most standard aquariums up to 75gal. Most are 18-24 inches tall), of "full spectrum" "daylight" (5000-7000K)

Led's tend to all point down so they need fewer watts per gallon usually, and depending on whether they have lenses to focus the light you might get away with even less wattage. So roughly I'd say .75-2watts of led lighting, (again possibly more if the tank is exceptionally tall).

I'ts not super scientific, but it has served me well. Usually with those guidelines in my head and past experience I can be fairly confident that what I'm thinking of doing will work, but never a bad idea to leave room for error where possible, like making sure there would be enough room on top of the tank for another light if you need it, or will bigger bulbs fit in your fixture, could you put lenses on some of the LEDs to get more punch down to the bottom of the tank, etc...etc..


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## Pubfiction (Feb 3, 2013)

Not in this hobby because many lights don't even list the lumens so no one could figure it out anyway.


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## Ray (May 12, 2009)

Pubfiction said:


> Not in this hobby because many lights don't even list the lumens so no one could figure it out anyway.


True, but one can get pretty reasonable guesstimates.

CFL - about 75 lumens/watt (true wattage, not "compares to" BS)
T12 - 40
T5/T8 - 90
LED - can vary, but 90-100 is pretty common


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## PDFanatic (Mar 3, 2007)

I have a 7400 lumen led over a 75 gallon tank, I feel I could get really good plant growth with a lot less of a light!!


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## Dendro Dave (Aug 2, 2005)

PDFanatic said:


> I have a 7400 lumen led over a 75 gallon tank, I feel I could get really good plant growth with a lot less of a light!!


out of curiousity do you know the wattage for that led?

It wouldn't surprise me if you could get good growth with less. 2 10watt _GE_ daylight bulbs are around 500 lumen each, so roughly 1000 lumens. I use 2 in a strip light to light a 10gal  ...I can imagine 7.4 of those strip lights would more then adequately light my 75 gal. I lit that pretty good with a 4 foot shop light and 2 strip lights with 2 10-15 watt cfls in each strip.

The thing about LEDs vs T-5, 8 or 12, Cfl or compact fluorescents, is the light is all pointing down usually. It doesn't have to reflect and whatnot before it gets to the floor of the viv, like a typical light bulb, Fluorescent tube or spiral.


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## PDFanatic (Mar 3, 2007)

Dendro Dave said:


> out of curiousity do you know the wattage for that led?


It says uses 80 watts?


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## Dendro Dave (Aug 2, 2005)

PDFanatic said:


> It says uses 80 watts?


Ya that puts it in the 1 watt per gal guidelines for LED... Sounds good, got a pic of viv and light?


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## PDFanatic (Mar 3, 2007)

Viv is freshly planted so when it grows in I'll pull plants so it doesn't look so busy. The trio of pumilio in there prefer a lot of cover otherwise I never see them.


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