It is best to buy a rectangular tank that is at the very most three inches higher than it is wide. Tall narrow tanks are not good because there is too little water surface open to the air. A wider tank has more open water surface, and the water can take in more oxygen. This is important for the health of your fish. The water takes in oxygen form the air, and the fish breathe in the oxygen through their gills.
Fish give off a gas that is called carbon dioxide, just as a human does when they breathe.
The carbon dioxide given off by the fish must escape into the air through the surface water. If it does not the fish will suffocate. The slightest bit of metal in the water can be poisonous to your fish. Paints, soaps, and certain plastics and chemicals must also be kept from the tank. Limestone products such as marble chips, seashells, coral, and coral sand, should also be avoided.