I just installed a light fixture, which hold 2 4 foot long 34 watt t8 bulbs. It hangs 5" above the top of my 3 foot long 30 gallon tank. I intentionally installed it to hang above and over the ends of the tank so that the houseplants on either side of the tank would benefit from the light, as well as the plants in the tank. Is there a rough formula to estimate how much light actually makes it into the tank? If the fixture was resting directly on top of the tank, it would be over 2 watts per gallon; However the way I have it set up there is a lot of light coming out into the room, rather than into the tank.