Oxygen Levels in Aquariums
Oxygen is an aquarium 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 rots in the aquarium requires and therefore depletes oxygen. Snails, unhealthy or dead plants, decaying live rock and live sand, uneaten food, and algae, just to name a few.
Organic acids, proteins, and carbohydrates, which are adsorbed by activated carbon, can also reduce the oxygen level in the aquarium, if the carbon is used up.
Oxygen enters the water through gas exchange in two ways:
- Surface agitation
- 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).
Celcius degrees (Farenhiet)
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
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