Hang on, I'm interested, tell me more about that point: why do small
tanks need more watt/gallon? Because crowded plants shade each other more?
No. I'm not sure exactly why this is the case. It's something that has become
evident empirically: the watts per gallon rule breaks down with very small or
very large tanks.
It probably is partly due to flaws in the rule of the thumb, as well as the
nature of flourescent lights.
The longer a flourescent bulb is, the more efficient it is. So a 4' bulb
throws out more light per watt than a shorter bulb does.
And the rule of thumb -- watts per gallon -- doesn't take into account the fact
that height is the most important dimension. As tanks get bigger, they don't
get much higher. Once you're over 100 gallons, the tanks only get longer, not
See this article for more info:
Basically, it found that for tanks under five gallons, 8 watts per gallon may
not be enough, while for tanks over 100 gallons, 2 watts per gallon is too