1. It is not considered a good idea to keep aggressive fish with fish of other temperaments. You should keep peaceful fish in an aquarium with other peaceful fish to avoid any sort of fighting between the fish. Aggressive fish should only be kept with other aggressive fish.
2. Research the aquarium equipment you are looking at before you purchase it. Different sizes of aquariums require different pumps and heaters. Find out which products are best for your aquarium before you start ordering the wrong products.
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.