1. If you use your tap water, you should make sure to remove the chlorine from the water before you add it to the aquarium. Chlorine can be very harmful to some of the fish in your aquarium. You can purchase kits to remove the chlorine from your aquarium at many retailers online.
2. When keeping aggressive species of fish, it is best not to introduce peaceful species of fish among them. Aggressive fish will tend to bully the more peaceful fish. Make sure to thoroughly read about the compatibility of the selected fish for the aquarium.
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.