Article © Shane Linder, uploaded January 01, 2002.

The importance of dissolved oxygen levels in the aquarium is a topic that is not given much thought by most aquarists and is rarely discussed in the aquarium literature. However, many aquarists, whether they realize it or not, have lost fishes because dissolved oxygen levels in their aquarium have fallen too low. Ever seen a fish gasping at the surface for air or had a tank full of fish perish overnight for no apparent reason? If you have, there is a good chance that you have experienced oxygen problems. These problems can be particularly prevalent in planted aquaria when, at night, plants become the major consumers of oxygen in the aquarium.

Water is an oxygen poor environment and contains only five percent of the oxygen that the same volume of air does. Dissolved oxygen (DO) refers to oxygen gas that is dissolved in water. Fish "breathe" by absorbing oxygen directly from the water into their bloodstreams using their gills. Studies have shown that a concentration of 5 mg/L DO is necessary for optimum fish health and that fish become distressed when DO falls to 2-4 mg/L. Fishes incapable of taking atmospheric air will perish when DO concentrations fall below 2 mg/L. Water, depending on chemical composition and temperature, can only hold so much DO. The maximum DO a given amount of water can hold is referred to as saturation. Saturation is a key concept for aquarists. An aquarium heated to 90F is saturated at 7.4 mg/L while an outdoor pond at 45F can hold 11.9 mg/L. Since fish are cold blooded, aquarists are stuck with a double jeopardy situation because the metabolic rate of fish is directly related to the temperature of the water. This means that at warmer temperatures, the water will hold less DO while fishes in the same environment will demand higher DO levels. Saturation is also effected by salinity (freshwater holds more oxygen than salt water) and altitude. Saturation decreases at altitude due to the fact that there is less pressure. While the effects of altitude will not be a major problem for most aquarists, I found it to be a major problem while living in Bogota, Colombia where my home sat at over 8,000 feet above sea level.

Many other factors affect DO levels that are important in the aquarium. Emmens, in Axelrod's Mini-Atlas, states that, "Plants make little difference, as we have seen, because they don't much affect oxygen supplies". Nothing could be farther from the truth. Of the three ways in which oxygen enters the aquatic environment: diffusion from the atmosphere, wind and wave action, and photosynthesis, photosynthesis by aquatic plants and phytoplankton are considered the most important. Other important factors affecting DO levels, from the aquarist's point of view are organic waste, decaying organic matter, and certain chemicals, such as formalin, that directly remove oxygen from the water.

If you have been an aquarist for very long, there is a good chance that you have seen fish either perish or become stressed by low DO levels. Signs of low DO include: all fish die at the same time (usually at night or in the pre-dawn hours), distressed fishes gasping at the surface, and fishes dying with their back arched, gills flared, and mouth open. This condition is most often seen in heavily planted tanks for reasons cited below. The worst thing an aquarist can do in these circumstances is medicate the fish. Too many aquarists have assumed gill flukes or another disease as the cause and immediately medicated. Most medications contain chemicals that remove oxygen from the aquarium and thus make the problem worse. The first, and correct, reaction is to immediately perform a water change.

Planted Aquaria
While, during hours of daylight, plants are the most important means by which oxygen enters the aquatic environment at night they are the enemy. The cycle by which plants add oxygen during daylight ours and consume oxygen at night is called the diurnal oxygen cycle. Oxygen levels begin rising at dawn, or when the aquarium lights come on, and steadily increase throughout the day. As the sun sets, plants stop producing oxygen via photosynthesis and begin consuming it instead. For aquarists that maintain heavily planted tanks, the importance of this cycle should be obvious. It is even more important in aquariums with CO2 systems. If you have a heavily planted aquaria, keep a good eye on it right before the lights come on. If you notice that the fish are sluggish or staying near the surface, there is a very good chance that they are suffering from low dissolved oxygen levels.

So what are the aquarist's options in heavily planted tanks? Probably the single best thing to do, without limiting fish selection, would be to hook up an air pump with one or two airstones and put it on a timer opposite your photoperiod. This will assist both the fish and the plants to breath better when there is no light. Planted aquarium aficionados can also over come the negative effects of the diurnal oxygen cycle by selecting fishes that are cable of taking atmospheric air such as Corydoras catfishes and Labyrinth fishes such as bettas and gouramis.

Oxygen exchange
Besides plants, oxygen enters the aquarium through diffusion and wind and wave action. Since diffusion takes place at the water's surface, the larger the surface area of the aquarium, the more diffusion will take place. Since a 15 gallon aquarium and a 20 "high" aquarium have the same amount of surface area (both tanks measure 12" wide by 24" long) they can both hold the same amount of fish. When buying a new aquarium, think about surface area and not gallons. Although at first wind and wave action might not seem to be related to the aquarium, they certainly are. Powerfilters, airstones, powerheads, and skimmers all act as wind and wave action. All of these devices move water from below the surface, where oxygen exchange is not taking place, to the surface where it can release CO2 and absorb oxygen. These devices thus act as the wind and waves in our little glass encased ponds.

Oxygen Thieves
Organic waste, decaying plant matter, and certain chemicals also steal oxygen from our fish. When aquarists think of organic waste, we usually think of the dangers of ammonia and nitrite poisoning. That fact is, that in addition to being potentially toxic, these compounds are also stealing the very oxygen your fishes need to breath. So, if you needed an additional reason to perform large frequent water changes, now you have one. Decaying plant matter mainly affects planted aquariums and it should go without saying that dead and dying leaves need to be removed frequently. However, even in non-planted aquariums most of us decorate with driftwood. Check your driftwood every few months. If you can easily scrape chunks off with your fingernail, you may want to replace the piece. Several chemicals that are common in aquarium medications also rob the aquarium of oxygen. Make sure that extra aeration is added during treatment with these chemicals (most manufacturers will recommend you do this in their instructions) and that, when treatment is concluded, several large water changes are done over a short period to remove any chemicals that may still be present.

Hopefully, this article will get aquarists thinking a little more about dissolved oxygen and its importance in the aquarium. After reading this article, you now understand how oxygen enters the aquarium and the factors and causes associated with oxygen depletion. It should also be apparent why scientists refer to dissolved oxygen as the single most important measure of habitat quality. If you think about it, maintaining good dissolved oxygen levels is just basic fishkeeping. Keep tank stocking levels reasonable, use adequate filtration, and keep on top of water changes. Do these things and your fish will breath much easier.

References

• Axelrod, H., W.E. Burgess, and C.W. Emmens. (1987) Dr. Axelrod's Mini-Atlas of Freshwater Aquarium Fishes. TFH Publications, 992 pp.
• Francis-Floyd, R. (1992) Dissolved Oxygen for Fish Production. Fact Sheet FA 27. Institute of Food and Agricultral Sciences, University of Florida.
• Mills, D.(1984) A Fishkeeper's Guide to Community Fishes. Tetra Press, 117 pp.

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