Dissolved oxygen
The amount of oxygen present in the water. Essential for the respiration of fish, corals and filter bacteria.
Detailed explanation
Dissolved oxygen is one of those parameters that almost no one measures but which can make the difference between a thriving aquarium and one that is struggling.
Fish breathe oxygen through their gills. Biological filter bacteria consume oxygen to process ammonia. Corals breathe oxygen at night when photosynthesis stops. They all compete for the same dissolved oxygen.
The concentration of oxygen in water depends on the temperature (cold water contains more), salinity (fresh water contains more than salt) and gaseous exchange on the surface.
The skimmer is the main oxygen supplier in marine aquariums. The microbubbles create a huge exchange surface. In freshwater aquariums the surface movement of the water and the air diffusers do the same job.
In summer the risk of deficiency increases. Warm water holds less oxygen just when the fish's metabolism is at its highest. In ponds, summer die-offs are almost always due to a lack of oxygen, especially at night when the plants consume oxygen instead of producing it.
An overpopulated aquarium or one with an insufficient filter can have chronic oxygen problems. Symptoms include fish gasping at the surface, listless fish, faded colours and abnormal bacterial growth.
The simplest solution is to increase gas exchange: more surface movement, an aerator, or simply lower the water level an inch so that the return of the filter creates more turbulence.
Reference values by aquarium type
| Aquarium type | Recommended range |
|---|---|
| Marine (reef) | 6 - 8 mg/lBelow 5 mg/l the problems begin |
| Tropical freshwater | 5 - 8 mg/l |
| Coldwater / pond | 6 - 10 mg/lCold water holds more oxygen |
| Brackish | 5 - 7 mg/l |
How it is measured
Drop tests exist (JBL, Sera) but they are not widespread. Digital dissolved oxygen probes are expensive but accurate. In practice most aquarists rely on symptoms rather than direct measurement.
What happens when it is too low
Fish gasping at the surface. Filter bacteria that slow down resulting in ammonia buildup. Corals closing. In severe cases sudden death especially at night.
What happens when it is too high
Virtually impossible under normal conditions. Excess oxygen (supersaturation) can only occur with forced injection systems and can cause air embolism in fish.
Coralia Lab monitors this parameter with AI diagnostics. Tank Score alerts you when a value moves outside the recommended range for your aquarium.
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