Dissolved Oxygen and Aeration in the Aquarium
Why dissolved oxygen matters for fish and beneficial bacteria, how it enters water through surface exchange, what lowers it, and how to recognize and fix low oxygen.
Why dissolved oxygen matters
Dissolved oxygen is the oxygen carried in the water, usually measured in milligrams per litre (mg/L) or as a percentage of saturation. Fish take it up through their gills, and the beneficial nitrifying bacteria in the filter also need oxygen to process ammonia and nitrite, so adequate dissolved oxygen is essential for both the livestock and the biological filter.
How oxygen enters the water
Oxygen enters the water mainly by gas exchange (diffusion) at the surface and, in planted or algae-rich tanks, by photosynthesis during daylight. Surface diffusion is relatively slow, so the rate of exchange depends heavily on surface area and movement. This is why air stones and bubbles help mainly by agitating the surface and increasing exchange, rather than the bubbles themselves adding much oxygen as they rise.
What lowers oxygen
- High temperature — warm water holds less dissolved oxygen than cold water; UF/IFAS notes seawater holds about 6.2 mg/L at 32 deg C versus about 10.7 mg/L at 4 deg C.
- Overstocking and overfeeding — more animals and more decaying organic matter consume more oxygen.
- Night-time respiration — plants, animals and bacteria all consume oxygen, and at night photosynthesis stops while respiration continues, so oxygen falls.
- Decay and bacterial blooms — decomposition of waste and dense bacteria draw down oxygen.
- CO2 injection and some medications — these can reduce available oxygen or stress the fish.
Signs of low oxygen
Fish gasping at the water surface, gathering near the filter outflow, and rapid or labored gill movement are classic signs that dissolved oxygen is low. Low oxygen (hypoxia) stresses aquatic animals and, if severe, can be fatal. These signs warrant prompt action to increase aeration and reduce the oxygen demand.
How to increase oxygen
- Increase surface agitation with the filter return, a spray bar, an air stone or a sponge filter.
- Lower the temperature toward the species' normal range, since cooler water holds more oxygen.
- Reduce stocking density and avoid overfeeding to lower oxygen demand.
- Improve circulation so oxygen-rich surface water mixes through the tank.
- Be cautious with CO2 injection and dose medications with extra aeration.