Why do some fishes need air pump in the aquarium and the others don't?

Air pumps are used to provide aeriation in the fish tank. Fish need oxygen in the water to live. Oxygen is also important for ensuring the breakdown of wastes that accumulate in the water. If too many of these wastes build up, the tank will become a toxic environment and the fish will die. So to keep your fish alive and healthy an air pump and filtration system is needed. In the natural world this aeriation of the oxygen is carried out by the process of photosynthesis. So one way that could prevent fish from dying without an air pump in a fisk tank would be to use live plants. Even with an air pump live plants can be used. As well as providing oxygenation this will also give your fish a nice treat. Goldfish love to eat pond weed. This can be bought at most good pet stores and garden centres. Just makesure the weed is not gathered with a lead clip/weight. If it is remove this before placing it into the tank. I would suggest using an air pump and filter though.