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. 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.