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Like all animals, oxygen is essential to the survival of fish. Fish require oxygen to breathe and bacteria require oxygen to break down wastes. The consumption of oxygen by nitrifying bacteria will depend on the amount of wastes entering the system. Oxygen enters the water primarily through direct diffusion at the air-water interface and from aquatic plant photosynthesis. However, most indoor aquarium systems lack sufficient photosynthesis. Therefore, mechanical means of aeration is the only alternative for supplying oxygen to aquatic animals maintained in these systems. Providing surface agitation to aquarium water through a series of diffusers (airstones) with a small airlift pump or compressor will increase the oxygenation of the water. Other methods of oxygen injection include spray/degassing towers and venturi injection (suction of air under water pressure). Aquarium systems generally require some form of aeration or surface agitation to maintain dissolved oxygen at safe levels.
Oxygen is not as abundant in water as it is in air. Oxygen makes up about 20% of the air we breathe, but dissolved oxygen, which fish absorb through their gills, represents only about 0.0001% of water. In fact, the maximum amount of oxygen that water wants to hold is only about 8.3 mg/L at standard temperature and pressure. This maximum level of 8.3 mg/L is referred to as oxygen saturation, which is influenced by temperature, elevation, and salinity levels. As these levels increase, oxygen saturation decreases. The oxygen measurements are best taken with a properly maintained reputable electronic oxygen meter with temperature and both percent saturation and milligrams per litre (mg/L) measurements.
Oxygen levels can change dramatically over a 24-hour period and should be checked at various times of the day and night, with particular attention paid to times of known low oxygen. Levels are usually lowest in the morning and highest in late afternoon. Warm water is much less capable of holding oxygen gas in solution than cool water. This physical phenomenon puts the fish in double jeopardy because at high temperatures the metabolic rate is increased, hence their physiological demand for oxygen is increased. Therefore, dissolved oxygen in an aquarium must be maintained above levels considered stressful to the fish. Usually larger fish are affected by low dissolved oxygen before smaller fish are.
The minimum dissolved oxygen level that rainbowfishes can safely tolerate depends upon individual species and temperature. As a general rule of thumb, levels should be maintained above 5.0 mg/L. When dissolved oxygen concentrations drop below 2.0 mg/L, rainbowfishes will be severely distressed, and when concentrations fall below 1.0 mg/L, they will begin to die. Prolonged exposure to non-lethal, low levels of dissolved oxygen constitutes a chronic stress and will cause rainbowfishes to stop feeding, reduce their ability to digest food, and make them more susceptible to disease.
© Copyright Adrian R. Tappin
Updated October, 1999.
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