Oxygen levels for fish and plants in a tropical aquarium

by Owen James on January 6, 2009

Bubbles of oxygen appearing on well-lit aquarium plants (also called pearling)

Bubbles of oxygen appearing on well-lit aquarium plants (also called pearling)

(Photo by: nttrbx)

Oxygen is an essential of life for virtual all organisms. People often forget that plants need to breathe, just like fish and other animals. Even filter bacteria need oxygen to survive.

Oxygen in the aquarium comes from two sources:

  • From the air, exchanged at the water surface
  • Oxygen is also produced by plants in sufficient light, via photosynthesis

What is the optimal oxygen level?

Fish and plants thrive at an oxygen level of 5-7mg of oxygen per litre of water. This is easily achieved via surface exchange provided your tank is not over-stocked with fish.

Healthy plant growth will also ensure a good supply of oxygen to the aquarium and makes for a more natural environment, though such plant growth is not essential.

Testing the oxygen level

Various test kits are available for testing the oxygen level in your aquarium. They aren’t required for normal situations, but can be useful in certain applications, such as when growing on a lot of fish in one tank as part of a breeding project, or if you’ve a densely planted tank where the oxygen level rises steadily over the day due to photosynthesis.

Insufficient oxygen

Even without a test kit you sometimes see fish gasping for air in badly kept tanks – a sure sign of oxygen deficiency.

Causes of oxygen deficiency include:

  • Too many fish for a given size of aquarium
  • Poor tank maintenance, including clogged filters and large quantities of rotting material
  • Poor surface ventilation
  • Dense but stunted plant growth

Provided the aquarium surface is exposed to a well-ventilated airspace, oxygen will enter the water if there is a deficiency compared to the atmosphere. However if the fish or other life forms in the tank use it up more quickly then it enters, the oxygen level will drop. Low levels of oxygen will threaten your fish and could potentially even impede your tank’s filtration, further reducing the water quality.

In the short-term, increase the surface water turbulence without delay using an airpump or filter. Then get to work on a long-term solution – reducing overcrowding, increasing plant growth, and improving tank filtration and maintenance.

Surplus oxygen

Oxygen levels above 9mg a litre are only ever likely to be seen in densely planted tropical freshwater aquariums that are extremely well-lit. In such tanks, photosynthesis increases the level of oxygen in the water far faster than the fish and other life forms can use it.

Consider reducing the lighting levels to move the tank towards a better balance between fish and plant life. (Adding more fish than the tank can support is not recommended, as you can still overload the filters with nitrogenous waste).

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