Aeration and Circulation

Aeration and circulation of the water is one of the most essential functions in aquarium systems. Water must be aerated to maintain adequate dissolved oxygen concentrations for fish and for proper functioning of the biological filter. Fish utilise oxygen dissolved in the water and release carbon dioxide and since the biofilter also utilises oxygen, a successful maintenance system requires a method to aerate or oxygenate the water. The amount of oxygen consumed by the fish is a function of fish size, feeding rate, activity level of the fish, and temperature. Consumption of oxygen by the biofilter bacteria is most closely related to the amount of ammonia entering the filter.

Aeration, circulation, and filtration are usually performed at the same time by a well-designed filtration unit. Aeration is usually applied in the fish culture tank and again prior to or within the biological filter, that portion of the recirculating system where organic waste products are broken down through bacterial decomposition. Trickle filters and revolving biofilters are designed to be self-aerating. Vigorous aeration of submerged filter beds is not recommended because beneficial bacteria can be dislodged from the substrate decreasing the filter's effectiveness. Air lift pumps are often used to move water through the tanks, accomplishing both aeration and pumping. Super-intensive systems may use pure oxygen injection although this is seldom used in the aquarium hobby. The level of aeration should be sufficient to sustain dissolved oxygen levels above 5.0 mg/L throughout the system.

In an aquarium, a number of functions are performed by aeration or circulation:

1. It adds oxygen directly to the water (oxygenates).
2. It circulates or mixes the water top to bottom to ensure that the oxygen and temperature levels are uniform throughout the system.
3. It moves aerated water away from the immediate area around the filter while dragging in unaerated water rather than retreating the same water.
4. Circulation encourages other harmful gases and excess nitrogen and carbon dioxide to escape to the atmosphere.

Diffused Air (Airstones)

Diffused aeration means the injection of air under pressure below a liquid surface to form bubbles. Oxygen diffuses across the bubble surfaces as they move toward the free surface. The oxygen transfer efficiency from air diffusers is a function of the bubble size from the diffusers and the way in which they are installed (loose or in airlifts). The aeration effect is usually localised around the diffuser. An airlift improves mixing and helps generate circulation. The finer the bubble the greater the efficiency and reducing bubble size from 5 mm to 2 mm can increase the mixing of the water and oxygen transfer efficient by almost 5 times. Therefore, bubble size is an important parameter affecting the oxygen transfer efficiency. Bubbles in this size range are usually produced by injecting air through devices having very small pore sizes <40 microns. Because of the very small pore sizes involved, such devices are often subject to clogging, resulting in deterioration of transfer efficiency and increases pressure of the pumping device.

The size of the bubble is a function of the quality and chemistry of the water. In seawater, or water with a high ionic content, the bubble size can be smaller than 1 mm, as the water becomes fresher, the size of the bubble will increase to around 3 mm in clean freshwater. If the level of fats and lipids is very high, the lipids will tend to reduce water surface tension and increase the size of the bubble.

Air diffusers are usually a solid stone type diffusers or a flexible membrane type diffuser. High levels of carbonates and iron present in the water can foul solid diffusers. If the water is very hard, with an alkalinity over 400 mg/L as calcium carbonate, then the carbonate will tend to come out of solution and form a precipitate on the diffusers. The carbonate deposit will block solid type diffusers. Flexible membrane diffusers are usually resistant to this type of fouling because the surface of the diffuser is flexible, and hard carbonates will tend to crack off the surface. If solid diffusers such as the stone type become fouled by carbonates they can become very difficult to clean. Air diffusers connected to an air pump or compressor are often used in culture tanks for live-food production, egg incubation and larval rearing. The bubbles they produce help in maintaining the oxygen levels and circulation. The porous material of the diffusers should be periodically cleaned, since small particles (algae, food residues, faeces) in the water can easily clog the pores and reduce (or even block) the air/oxygen flow.