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. It is a good idea to keep carbon on hand in case of emergencies. Carbon can be used to soak up anything harmful that may be introduced into your aquarium. So keeping an emergency stock can be a life saver.
3. 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.