Please write back soon. It addresses the fish-only tank as well as the reef tank. The difference being that reef tanks are designed to keep invertebrates as well as fish. They tend to have sophisticated lighting, filtration, and water movement techniques. Fish-only aquariums are mostly simpler, less expensive tanks than reefs.
Fish diseases are well understood and treated.
Invertebrate diseases are at this point largely unknown. Both are highly rewarding. But in somewhat different ways. You're not likely to be disappointed either way you choose. Best place to begin is finding a good aquarium shop.