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In addition to tank depth, there are some other factors that come into play with regards to tank size. People have stated for a long time that large tanks need relatively less light and that small tanks need relatively more light than aquariums of standard size (20g-60g). This link is an interesting analysis of the lighting that Amano has used in various size aquariums. It is an imperfect analysis, but it demonstrates to me that large tanks can get by on far less light than smaller ones, even if there is a greater average distance from light source to substrate in bigger aquariums.

How could that make sense?

It probably has something to do with the ratio of volume to footprint. Watts per square foot of surface area is probably a better measure of lighting requirements than watts per gallon.

I've posted this here before, but I think it's interesting to consider:

A standard 10g aquarium has a footprint of 20" x 10" or 1.39 square feet. 30 watts of light over this tank (3 watts per gallon) would provide 21.6 watts of light per square foot.

A 180g aquarium has a footprint of 72" x 24" or 12.0 square feet. 540 watts of light over a 180g tank (again, 3 wpg) would provide 45 watts per square foot, more than twice as much light.

Go to the extreme:

A 2.5 gallon tank is 12" x 6" or 0.5 sq feet. 7.5 watts / 0.5 sq feet = 15 watts per sq. foot.

A 300g tank is 96" x 24", or 16 sq. feet. 900/16 = 56.3 watts per sq. foot. The difference now is a factor of 3.75.

I think this partly mitigates for the dropoff in intensity that happens as the substrate gets further from the light source. The 2.5g aquarium is only 8 inches deep while the 300g one is 30 inches deep.

So, after all that, it remains a complex issue. For the most part, lighting is a huge variable that is determined only once at the time of initial setup. To establish a well-balanced system, everything else in the tank must be tuned to match that parameter.
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