The Importance of Dissolved Oxygen

The Importance of Dissolved Oxygen

Oxygen, specifically, oxygen gas (O2), is as critical to life in the aquatic environment as it is to life on land. Available oxygen is often the limiting factor in the success or failure of an aquarium. Two systems in the aquarium place the greatest demand on available oxygen – the animals and the biological filter. Absorption through gills and other structures introduces oxygen to animal tissues making it available for the biochemical reactions that sustain life in fish and invertebrates. The amount of oxygen available to fish and other aquatic animals is often the limiting factor in stocking capacity.

Biological filtration, carried out by aerobic bacteria in the filter as well as every surface in the aquarium or pond, places an incredibly high demand on available oxygen – much higher than the animals themselves. Temperature, salinity levels, medication, and elevation are the four factors that will affect the amount of oxygen available in water. The amount of oxygen available to fish and other aquatic animals is often the limiting factor in stocking capacity.

Oxygen gas enters and leaves aquatic systems at a fairly constant rate over time. Oxygen gas that enters an aquatic system is referred to as dissolved oxygen and is the form of oxygen available to aquatic animals and bacteria. Temperature, fish load, stress, over feeding and medications have a direct, and sometimes disastrous, impact on the amount of dissolved oxygen in the aquarium. Elevated temperatures, for example, reduce gas exchange rates and the water has a lower capacity to retain soluble oxygen gas. The situation is further complicated by increased metabolic rates which increase the overall demand for oxygen. The final assault is stress behavior which leads to decreased immune response and onset of disease.

Increasing Dissolved Oxygen

Mechanical filtration constantly breaks the surface of the water exposing more of it to the air. Mechanical filtration, such as powerheads, box filters, and, to a lesser extent, canister filters, create a waterfall effect which breaks the surface and increases the rate of gas exchange between the air and the aquarium water. Trickle filters driven by high capacity pumps, achieve the highest rates of gas exchange. The high surface area filter media, typically supported in a chamber rather than submerged in the aquarium or pond water, is ideal for maximum colonization by beneficial bacteria which are constantly bathed in oxygen-rich water.

Careful observation and regularly scheduled water tests give a good indication of the overall integrity of the aquarium or pond and serve as early warning systems for impending problems that lead to reduced oxygen availability. Lethargy, loss of appetite and gasping at the surface signal acute problems that require immediate intervention, beginning with partials water changes. Front-end and long term solutions for maximum dissolved oxygen levels include:

  • – Be observant
  • – Incorporate mechanical filtration, ideally, trickle filtration
  • – Stock the aquarium or pond conservatively
  • – Remove dead plants and animals immediately
  • – Avoid overfeeding
  • – Perform regular maintenance
  • – Test the water on a regular basis – without fail

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