The importance of oxygen in the aquarium

Aquarium goldfish

How important is oxygen in the aquarium? This is a subject that is not often discussed or taken under consideration.

Obviously, fish need oxygen to breathe, plants need oxygen at night and the beneficial bacteria need oxygen to break down (oxygenate) waste. Basically everything that dies off or decays in the aquarium requires and therefore depletes oxygen. Unhealthy or dead plants, decaying live rock and live sand, and uneaten food just to name a few.

Clear cloudy aquarium water, remove nitrates and simplify aquarium maintenance

Organic acids, proteins, and carbohydrates can also reduce the oxygen level in the aquarium.

Oxygen enters the water through gas exchange in two ways:

  1. Surface agitation
  2. Plant photosynthesis

The amount of oxygen that can be dissolved (saturated) in the water is dependent on the water temperature and salinity levels. Increasing temperatures and salinity will allow less oxygen to be saturated in the water.

The following table shows saturation levels at different temperatures.

Salinity is measured in ppt and the dissolved oxygen content in mg/l (ppm).

C (F) 0 5 10 15 20 25 30 35
18 (64.4) 9.45 9.17 8.90 8.64 8.38 8.14 7.90 7.66
20 (68.0) 9.08 8.81 8.56 8.31 8.06 7.83 7.60 7.38
22 (71.6) 8.73 8.48 8.23 8.00 7.77 7.54 7.33 7.12
24 (75.2) 8.40 8.16 7.93 7.71 7.49 7.28 7.07 6.87
26 (78.8) 8.09 7.87 7.65 7.44 7.23 7.03 6.83 6.64
28 (82.4) 7.81 7.59 7.38 7.18 6.98 6.79 6.61 6.42
30 (86.0) 7.54 7.33 7.14 6.94 6.75 6.57 6.39 6.22

Please note that a salinity of 0 corresponds to a freshwater tank.

For example, a freshwater aquarium with a temperature of 75 F can dissolve 8.4 ppm of oxygen. A saltwater tank with a temperature of 82.4 F and a salinity of 15 ppt can dissolve 7.18 of oxygen.

The above levels show 100% saturation, the maximum possible. The average saturation in an aquarium is about 70%.

The level of dissolved oxygen varies throughout the day, it is higher during the lighting period and has the lowest concentration in the early morning hours.

A dissolved oxygen content of 5-7 ppm is sufficient, first signs of stress will show if the content drops below 4 ppm, fatalities can be expected at 2 ppm.

In general, an aquarium has sufficient oxygen to support the livestock. Nevertheless, as indicated in the table above, higher temperatures or salinity levels might need attention and a good source of oxygen.

Some reasons for low dissolved oxygen levels are:

  • a short lighting period in planted tanks, plants produce oxygen during the lighting period and use up oxygen at night
  • an overstocked tank, which translates to larger amounts of waste created that in return requires more bacteria to oxygenate the waste
  • no or too little water agitation
  • waste rotting in the filter or in the gravel

Another term in connection with dissolved oxygen is the Redox (reduction-oxidation) potential.

In simple words Redox refers to the relation of oxygen and waste particles. The more waste, the less Redox potential due to lesser oxygen. Redox can be measured in mV and the range should be between 150 to 250 mV.

Redox is also related to the biological oxygen demand (BOD). BOD is the measurement of how much oxygen is needed to break down the waste created. The higher the BOD value, the worse the quality of the water.

An acceptable BOD value is at about 1-2 mg/l (ppm).

Nitrates contribute to high BOD levels, since this generally indicates a high break down rate of waste.

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12 thoughts on “The importance of oxygen in the aquarium

  1. Ross Lindsay says:

    I went on vacation for 10 days where the temperature in my room with my 7 ft tank was in the 90’s. During that time around 15 African cichlids died – average size around 3-4 inches, but the 2 similarly sized American cichlids did not. I did notice the water level in the sump was about half after returning. The person looking after the fish did do some cleaning in the room with chemicals. Any suggestions on the fatalities?

    • Thilo
      Thilo says:

      As the water temperature increases, the oxygen levels decrease. A low oxygen level by itself can cause issues, but can also be a contributing factor if the water quality is not near perfect. Keep in mind that if the water levels get lower due to evaporation everything that remains will have a bigger impact on the system. Organics, inorganics, ammonia, nitrates etc all are getting more potent as these substances become less “dissolved”.

    • Scott @ Algone
      Scott @ Algone says:

      Algone can be used in both, a wet-dry and a sump. Simply drop the entire pouch (without opening it) in either one. Algone fits most filter systems, or can alternatively be placed anywhere in the system provided it is submerged in water and receives adequate water flow. The water flow and of course ease of use are the primary reasons we recommend placement in the filter or sump. See our instructions here for more information.

  2. Dusan says:

    Just noticed I didn’t specify aeration. For the whole life of this aquarium aeration is by a strong air pump connected to two devices in aquarium: one is standard air stone producing stream of small bubbles and the other is like reverse vacuum cleaner suction end oriented upwards-wide in one direction, narrow in another, resulting in a few inches wide stream of large bubbles released near the surface. Both aeration elements alsoexhaust near the filter output and the combined effect is both bubbling and small wave action emanating from that area. Served the purpose well for a year and a half…should still do the same…

  3. Dusan says:

    You list some typical reasons for low oxygen in aquarium water however, I have encountered a conundrum with it not explainable by 4 listed typical problems. What else could becausing it?
    Aquarium in question is 1.5 year established and functioning well with perfect ammonia 0, nitrates 0, nitrates 0-10, pH 8-8.2 tested weekly. 20-25% water changes weekly. 77-79F T. Housing about 54cm worth of Mbuna (13 fishes of various sizes) and couple of plecos. No diseases or problems up to now. Intentionally overfiltered – oneEheim 2215 andone Eheim213, 9w uv filter 8 hrs/day. 50gal/200 l size.
    About month ago I noticed fishes breathing heavily which increased to them going to surface. First I attempted daily 20% water changes but as that did nothing, I moved fishes to another similar aquarium. Despitecongestion there, heavy breathing stopped-so not a disease. I did extremecleaning of affected aquarium:100% waterchange, extreme substratevacuuming, filter cleanups, all decor and two plants (Anubias) cleaned. Aquarium re-cycled. It did cyclefast, in 4 days. All parameters back to normal, water clear as usual… Returned fish in-immediately went back to lack of oxygen conditions. For now I am adding hydrogen peroxide and it helps big time. However I do not understandthe source of the problem. Any helpful explanation ideas?

    • Thilo
      Thilo says:

      Not really an issue with the dissolved oxygen, but ammonia/ nitrite poisoning of the fish. Anything that interferes with the beneficial bacteria’s ability to convert ammonia to nitrite to nitrate can cause ammonia and nitrite to spike. These spikes can be temporary but will cause dangerous levels.
      The signs of ammonia and nitrite poisoning are quite similar, heavy breathing, lethargic behavior, gasping for air, inflamed gills, less appetite. Nitrite is less toxic, but interferes with the oxygen carrying blood cells hemoglobin.

      You did all the right steps to safe your fish and to solve the issue.

  4. Twigy says:

    mbuna African cichild should be kept in large groups of 10-15 or more.
    No more than 10mm of gravel.
    And will require a 300 ltr tank when fully grown for this number.
    A 3000 ltr/hr filter is recommended.
    Near to perfect cycle of the tank required.
    The return spray bar can be directed at the top of the water to provide moderate surface agitation.

    • Scott @ Algone
      Scott @ Algone says:

      I assume you are referring to an ‘air pump’ since you are posting this under our Oxygen article?
      If that is the case, please note that no air pump will be required. They don’t really introduce ‘usable’ oxygen into the aquarium water. Oxygen primarily enters the tank through surface agitation. Simply assure that there is adequate water movement at the surface of your aquarium. This is easily accomplished with your filters return. Alternatively a powerhead can be used. Simply install a powerhead so it is aimed at the water surface, creating the desired movement needed to oxygenate the fish tank.

      • Matt says:

        This is not entirely true. Not to shoot down your theory, because you are correct, however a simple airline tube can create enough surface agitation to allow sufficent Oxygen to enter the water.

        • Scott @ Algone
          Scott @ Algone says:

          You are correct about the airlines creating surface agitation which provides some oxygen to the tank. Thank you for your contribution.