A couple of products that claim to have live bacteria are on the market, and you can buy chemical fixes for almost everything. However, none of that stuff will solve the underlying problem if it's related to over feeding, over crowding, incomplete cycling or species incompatibility. That covers the majority of aquarium problems. For some reason, it's easier to start with a single species and a rather sparsely populated, completely cycled tank. That at least gives the fish a healthy environment in which to adapt. Different species many times require different environments, and many popular species of aquarium fish become aggressive as they mature. Cheap, simple and slow combined with research usually lead to success. Of course, very few of us approach things that way.