1. It is generally a good practice to purchase the largest aquarium that you can actually afford. A larger aquarium will be less susceptible to quick changes in the chemistry, and will offer much more stability.
2. When adding particularly aggressive fish to your aquarium, it is a good idea to move around the rocks and such in the aquarium before placing the new fish into the aquarium. Moving the rocks in the aquarium will allow the fish to go back and stake out their own territory.
3. Some people go by the ìinch of fish, gallon of water rule when deciding on how large of an aquarium to buy. This is a completely false and bad method to go by, you should always check your individual fish's tank requirements.