We all breathe oxygen – even fish. Humans inhale air, and the oxygen in the air is transferred to the red blood cells in the lungs. Fish take in water, and the oxygen in the water is transferred to the blood when it passes through the gills. Aeration is the process by which oxygen is transferred from the air to the water. This is relatively easy to accomplish in a freshwater tank, a bit more difficult in a saltwater aquarium
In your fish tank, this happens in a few ways.
Since small freshwater tanks – 10 gallons and less – have little surface area, adding aeration through an airstone can be helpful. In larger freshwater tanks outfitted with power filters, the movement of the water caused by the filter will usually provide enough oxygenated water without an airstone, although the stream of bubbles from the stone can be pleasant to watch and certainly won't be doing any harm.