As a general rule, marine animals are adapted to waters with high levels of dissolved oxygen. Ocean environments are usually well oxygenated by the movement of water and waves and by the large surface area of water that is exposed to the air. Exceptions exist, as in the case of isolated bays and lagoons that do not experience a great deal of water circulation. Freshwater habitats vary more widely in terms of dissolved oxygen levels. Some fresh-water fish, such as the lungfish, obtain most of their oxygen from the atmo-sphere, not from the water.

Aeration occurs when oxygen from the air enters the water at its surface. That fact may, in some cases, influence the shape of aquarium that you want to buy. A long, low aquarium will allow for a greater exchange of oxygen than will a high, narrow aquarium, despite the fact that both may hold the same amount of water. Of course, mechanical aeration can be used to overcome the limitations of tank shape.

The exchange of oxygen is facilitated by water movement. In the aquarium, this movement is created by the flow of water exiting from the filter or by the bubbles that arise from air circulated by a motorized pump through porous aerating devices known as “air stones.” The oxygen that is contained in the bubbles created by air stones or other forms of mechanical filtration is lost to the air and does not add to the dissolved oxygen content of the water. By disturbing the water surface, these bubbles and other forms of water movement create turbulence, which allows atmospheric oxygen to dissolve into the water.

When considering how to aerate your aquarium, bear in mind that many aquatic animals are adapted to quiet waters. These species will likely be stressed by strong water currents, because they are unable to feed or swim properly under these circumstances. On the other hand, fish from areas of strong water movement, such as coral reef species, generally require strong aeration and are quite comfortable with fast-moving water. Such species will generally have high oxygen requirements and will perish without proper aeration. Most filters on the market today allow you to control the rate of water outflow into the aquarium, thus enabling you to tailor water turbulence to the nature of the animal that is being kept. For those species that require less water movement, air stones operated by small pumps may be used to provide oxygenation without disturbing the animals.

The ability of water to hold dissolved oxygen is affected by the temperature of the water. Cool water is able to hold more oxygen than warm water. For this reason, be sure to research carefully the habitats from which your pets originate, as those from temperate waters will generally have higher oxygen requirements than will those from tropical waters. Again, there are exceptions to this rule, as in the case of warm-water fish that nonetheless live in areas of moving water that are rich in dissolved oxygen.

You should also keep in mind that the beneficial bacteria in your aquarium and in the filter materials are aerobic, meaning that they require oxygen to survive. The flow of oxygenated water through these materials must be of sufficient strength to support the beneficial bacteria. It is also important that all areas of the filter bed are kept clean, so that water can easily flow through. The buildup of organic material and other detritus will block water flow, creating stagnant pockets where harmful anaerobic bacteria can take hold.