If you are going halide it's more off of the depth of the tank than (watts/gal). I overkilled on lights a long time ago and was running 2 400 watt lights on a 75 gallon. Glad I did since I'm upgrading to 180 now, but I believe they said 250 watt is good up to 24 inch deep then go to 400 watt. Of course this is all taken with a grain of salt and you decide what you really want.