It will be wise to learn something about fishes and their requirements if you plan on setting up an aquarium.
Fish take in water through the mouth; they do not drink it, however, but pass it out backwards through the gill-openings at the sides of the head, under the bony gill-covers. As it goes through it bathes the gills, which are so constructed that they can take oxygen out of the water and get rid of carbon dioxide in exchange. The gills, then, are their breathing organs; they have no lungs. But the process of breathing is otherwise much like ours, and if there is not a proper supply of oxygen in the water, they will sicken and die.
The oxygen is dissolved in the water, and the carbon dioxide given off, through contact with the air at the surface. Thus it will be understood that if the surface area is too small the water will become charged with carbon dioxide, and there will not be room for replacement of the oxygen when it is used by the fish.