A refractometer is always a very useful tool to have. If you're going to have corals in your tank then must-have test kits will be Calcium, Alkalinity, Magnesium and Nitrate. The others like Ammonia, Nitrite etc. I wouldn't really bother with if you have an LFS close by that can check them for you.Okay but what should I be testing for and what about a refractometer?