Agreed with the above. The most important medication you can give your fish is clean water, hands down. In order to maintain good water, you will need to have a decent-sized tank and an appropriate cleaning schedule. Doesn't get much simpler than that.
Some medicines will actually have adverse effects on fish. To fix the problem you need to eliminate the cause. So if your betta's fins are rotting because, say, you keep him in a cup and clean it once a week, dumping in medicine will do more harm than good. But if you go out and buy him a 2 gallon tank and a heater and keep it really clean, then put the medicine in, it will help.
Without knowing your tank sze and cleaning schedule the only definite advice I can give you is to stay away from BettaFix, MelaFix and BettaRevive.