I think the general is 2 watts per gallon. If it is an incandescent (which most tanks come stock with unless otherwise specified) it is not providing any usable light for your plants.I set up my fish tank and i realised that the plants are turning bit yellow they look burnt. I went to go and look at the light and its 11 watt. I have no fish at the moment but I am planning to get Siamese fighting fish.