It depends on what type of tank you want. For FOWLR, you'll want to test pH, S.G., ammonia, nitrites, and nitrates. There are several test kits out there that offer all of these in one package. Phosphates may be a useful test also if you have algae problems and/or are using tap water for the water source. If you want to add coral, alkalinity and calcium are very important also. Although generally not considered a "test," it's very important to monitor temperature no matter what type of tank you have.