Why is an aerator in a fish tank not working?

When a freshwater aquarium is not overstocked, the normal water movement provided by the filtration system will be enough. It is not necessary to bubble air through the water to 'aerate' it. This has very little effect on the oxygen content of the water because the bubbles are in contact with the water for only seconds before they reach the surface and burst; gas exchange occurs on the surface of the water itself. Aeration without a filtration system is pointless - as is trying to keep fish at all without a filtration system. The job of the filter is to process ammonia, which is constantly produced by fish and is highly toxic. Bacteria live in the filter media and remove the ammonia from your tank. This is called the nitrogen cycle and very few pet stores will tell you about it. For more information on how to 'cycle' your tank, try some fish forums. The members are always willing to help.