1. You should always research the different types of fish you are going to be housing together. Certain fish can be extremely aggressive and cannot be paired with other fish of different temperaments.
2. 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.
3. You must always treat the water in the aquarium before attempting to place fish. If you fail to treat the water the fish can die from the condition of the water.