The primary function of aquarium lights is to provide light for your fish. Of course, it's also a matter of aesthetic choice, but fish really do need lights to survive and grow. Fish need light to distinguish between day and night and need light to grow into full-sized adults. If you have ever seen a fish tank in the dark, you'll see that your fish are not moving around nearly as much as they do with light. Fish have a circadian rhythm as we have, and they need light to maintain it.