Depends on what you're keeping. Corals have requirements. Fish are ok with just salt/water changes, but you have to watch alkalinity: test and supplement required. I don't own copper and won't use it: there are better methods for the fish I prefer, and certain species should not be dosed with it, notably Angels, dragonets. My own rule is never to treat a well fish. Simply quarantine and observe. I must say I have seen only one sick fish in the last 10 years, and that one had a virus (lymphocystis) for which there is no treatment.
In general, have a refractometer, an alkalinity supplement, and a test for it: and if getting into stony corals, you also need calcium and magnesium tests and supplements.