Can You Have Too Much Oxygen in a Fish Tank?

Dissolved oxygen is an essential requirement for fish and invertebrate animals, such as sea anemones and sea stars. Beneficial nitrifying bacteria, which break down the metabolic waste of aquatic creatures, are equally dependent on oxygen. The natural limit of dissolved oxygen in aquarium water is termed the saturation level and the level is measured in parts per million (ppm).

  1. Saturation Level

    • Water is 800 times denser than air and contains 95 percent less oxygen. Fish expend a considerable amount of energy breathing but are able to extract 80 percent of the available oxygen from water, as opposed to humans, who extract only 25 percent of the oxygen they breathe in. At a given temperature, the saturation level of oxygen in water is in the region of 7 to 8 parts per million. Under normal conditions, there is never too much oxygen in a fish tank, unless the aquarist purposely attempts to super saturate the water. Aquarists can supersaturate aquarium water by diffusing 100 percent liquid oxygen into the water, but there is no real value to doing so.

    Temperature

    • The amount of oxygen that water can hold is temperature dependent. Warm water holds less oxygen than colder water. Aquarists who keep tropical fish need to ensure that their aquariums are correctly filtered, as uneaten and decaying food robs the water of oxygen. Tropical marine aquariums require dedicated care, because the aquarist is dealing with both warm and salt water. Powerful and high-quality filters are therefore always recommended for these types of tanks.

    Marine Versus Freshwater Aquariums

    • Sea water holds less dissolved oxygen than fresh water. The majority of marine hobbyists keep tropical instead of cold-water tanks, which compounds the problem of ensuring that the aquatic creatures receive sufficient oxygen. Filters, particularly mechanical filters, should be cleaned regularly, as the bacteria that break down the dirt and detritus trapped in this filter material consume large amounts of oxygen.

    Oxygen and Carbon Dioxide

    • Fish and marine invertebrates require oxygen to survive and release carbon dioxide into the water through the process of respiration. Carbon dioxide can't easily be released from a fish's blood stream when levels in the water are too high. Excessive carbon dioxide in the body prevents the blood from absorbing oxygen, and the fish will die. Oxygen enters the water and carbon dioxide dissipates into the atmosphere from the water surface. It is important to break the surface tension by aerating the water to allow for the effective exchange of oxygen and carbon dioxide.