Are air pumps needed for oxygenation?
Oxygen is of course essential for fish. However, it is not always necessary to have an air pump to provide oxygenation. It is a common misconception that water is oxygenated by the bubbles produced by an air pump. In fact, very little oxygen passes from the bubbles into the water.
The main site for gas exchange in an aquarium is at the water surface. A larger surface area clearly provides more area for oxygen to enter the water, but how does filtration or aeration help?
Consider a stream or river: it is well known that fast flowing waters are well oxygenated, while still waters are often stagnant. The greater the circulation and surface movement of water in an aquarium, the greater the oxygenation. More oxygen is added when the bubbles from an air pump break the surface than when they rise through an aquarium.
Most filters will provide adequate oxygenation as they circulate the water and cause surface movement. An added air pump will provide even greater oxygenation, and is a good safeguard should the filter ever stop working, but is usually not a necessity.
What is protein skimming?
Protein skimming does not fall into any of the previously defined categories of filtration, but it is a process whereby waste products are removed from the aquarium.
Protein skimming uses fine air bubbles to strip proteinaceous waste products from the water. You may have noticed the white froth that is sometimes formed by waves as they break on the beach. The waves force millions of tiny bubbles into the water and as these rush to the surface, they collect dissolved proteins, and the foam results. The same mechanism, although on a smaller scale, is used by protein skimmers in the aquarium. Air, from an air pump or venturi intake, is forced up through a column of water (the reaction chamber), and the foam is collected at the top.
The process of protein skimming can occur in both fresh and salt water. However, to get protein to foam out of freshwater requires an extremely large reaction chamber and very powerful pumps. Therefore, freshwater protein skimming is not practical for home aquaria.
Protein skimming is however readily achievable in marine aquariums, and is commonly used. Because protein skimming removes nitrogenous wastes before they start to decompose, it reduces the production of ammonia and so takes some of the load off the biological filtration. Secondly, it removes nitrogen without producing nitrate. This is particularly desirable in marine reef tanks, since nitrate is toxic to corals and many invertebrates.
Protein skimming on its own is usually insufficient filtration, but its addition to marine tanks is beneficial, particularly where invertebrates are to be kept.
The dangers of overfeeding
As aquarists we have all at some stage been warned not to overfeed our fish. Why is overfeeding so bad?
Fish food contains various carbohydrates and protein. Uneaten food in the water will be broken down by bacteria in the same way as the fishes waste products. The breakdown of carbohydrates consumes oxygen and produces carbon dioxide, while the breakdown of protein produces ammonia. The combined effects of oxygen depletion and ammonia can be lethal.
Clearly, with better filtration to remove ammonia and oxygenate the water, the risks of killing your fish by overfeeding are lessened. They are not, however, completely eliminated, so you should still take care.