Most first-time goldfish owners, especially parents who give their children goldfish, purchase small round bowls as aquariums. But goldfish need room to grow. Small spaces not only restrict growth, but make the fish more prone to illness. Owners should have a tank of at least 20 gallons for no more than 35 ounces of goldfish. As the number of fish grow in the tank, the dissolved and particulate organic carbon content of the water rises, causing the bacterial load in the water to increase. A crowded fish bowl or aquarium is a mishap waiting to happen. These aquariums should also have air pumps to keep the level of oxygen in the water suitable.