i really don't think there is such thing as too much light, however the more watts the more heat, and you don't want your tanke to get to hot. But it really does depend on what you are wanting to keep in it. As Andy said. The typical suggestion for a tank, if you are want to do corals and what not, is 3-5 watts per gallon. Some people use more ike 7-9 watts per gallon, and some people try to get away with less (but they are usually not as happy). But if it is fish only you don't need all that much light, and but the it doesn't harm most of the fish.