1. Having a quarantine tank on hand is a very good practice to use if you frequently buy new fish. The quarantine tank will weed out any sick fish that could possible get into your aquarium and cause harm to the other inhabitants.
2. It is important to pay attention to the limitations of the fish you place in the aquarium. Do not place fish into aquariums smaller than there recommended range, this can lead to increased aggression and decreased health.
3. Only use medication in your aquarium when you are out of other options. You should use a separate tank to medicate sick fish as some medications can have harmful effects on healthy fish.