Though there are other needs, seawater quality is the number one requirement for the success of a marine aquarium. Attention to seawater quality will pay dividends to the aquarist.
One of the necessities for high seawater quality is sufficient oxygen content, and there are different ways of insuring that the content is high.
Air pumps are an older type of technology, still in use today. Most air pumps run on a vibration principle, where a flexible arm is connected to a diaphragm. The arm is moved rapidly back and forth by electric energy, which in turn moves the diaphragm backwards and forwards. Air is sucked in on the backward movement and pushed down the airline on the forward movement. The air is usually released into the seawater through an air stone, though an air curtain can be used. The air curtain is simply a length of rigid airline with holes spaced equally along its length and blocked at one end.
Air pumps can be obtained in various sizes, the smaller ones with single air outlets and the larger ones with two. Maintenance on the air pumps is very simple: if there is an air filter fitted on the intake of the pump, the pad must be changed when dirty to allow the passage of air, and the only other consideration is the condition of the diaphragm. If this is showing signs of wear or splitting, it should be replaced with new. The running cost of the device is very low.
Air is pumped into the seawater and, as said, is normally released through an air stone or curtain. In marine use the air stone is normally a wooden one, which creates smaller bubbles. If the air stone is not releasing bubbles as readily as in first use then it should either be cleaned by a fresh water soak or, better, replaced.
Ok, back to oxygen then. It seems pretty obvious how the oxygen gets into the seawater, the answer being the bubbles. This is true but only partially. Some oxygen will get in from the bubbles, because each bubble is an air/water interface. However, the majority of the oxygen is taken in at the surface. This is because the rising stream of bubbles moves the seawater upwards in a column. When it reaches the surface, it peels away outwards in a circle and the seawater constantly passes this much larger air/water interface. It follows that the power of the air pump must be sufficient for the size of the aquarium, and also for its depth.
There is nothing wrong with using an air pump in a marine system. There are disadvantages though.
The first disadvantage is salt spray. When the bubbles are bursting at the surface they tend to throw spray about. After a short period of time salt encrustation can be seen on any wires, glass covers etc that are above or close to the bubble area. To avoid this as far as possible the aquarist should position the air stone so that the upwelling at the surface forms a mound. Experimentation with the size of this mound by moving the air stone up and down will indicate the best for minimum salt spray. If an air curtain is in use, the air curtain can be moved up and down. Of the two, for efficiency the air stone is probably best.
Some pleasing decorative effects can be achieved with a column of air within the aquarium, and the air stone can usually be hidden from direct view.
There is another problem, which is probably more applicable to reef systems than fish only ones. This is to do with seawater movement. It is well known that corals require good seawater movement for their well being, some corals more than others – hard corals generally need more than the soft variety. The problem is that this strong seawater movement could interfere with the air column coming from the air stone or curtain. At best, the column could be distorted and become less decorative. At worst, the aquarium could end up with air bubbles all over the place, not a particularly decorative or natural scene.
Overall, the need for an air pump to service a modern marine aquarium should not arise. The use of weirs and sumps greatly assists the intake of oxygen. Most of all, correct seawater movement in the aquarium will bring a constant flow of seawater to the surface, where a large air/water interface permits good gas exchange. The protein skimmer also assists with oxygen, though this must be considered as very much of secondary importance with this device.
So there shouldn’t be any need for an air pump. There is one place where this equipment can be of great use and that is with the bucket used for routine seawater changes. When the fresh water and dry salt are mixed, it can be left overnight with the air pump running to mix and oxygenate. Any spray from the bubbles doesn’t matter in this situation, particularly if the bucket has a loose fitting cover. The pump does not need to be particularly powerful for this purpose.
The air pump is not out of date. It is simply that a properly designed and stocked aquarium will have no need of one.