Oxygen (part 2)

In the first part of this diptych Oxygen (part 1) We've discussed what oxygen (gas) is, its purpose, how it enters the pond, and what quantities (absolute and relative) you need for optimal pond function. In this section, we'll look at how fish absorb oxygen and the possible causes and consequences of a deficiency.

Oxygen absorption by the fish

Fish absorb oxygen through their gills. The gill arches contain numerous gill plates, which together form a large surface area through which oxygen-poor blood flows. This blood is enriched with oxygen from the surrounding water through diffusion. Bony fish (including koi and goldfish) must continuously open and close their mouths to allow water to flow past their gills. Some fish (such as sharks and sturgeon) lack this mechanism and therefore must continue swimming (or lie in the current) to allow sufficient oxygen-rich water to flow through their gills.

A lack of oxygen in the pond

A lack of oxygen in the pond can have several causes:

The water is very warm, which means the oxygen standard cannot be met.
The pond does not have enough aeration or water movement to allow sufficient replenishment from outside air.
The water is heavily polluted, causing rotting processes to consume more oxygen than is produced.
There are many underwater plants present, which causes a lack of oxygen at night.
Oxygen-depleting agents, such as drugs, have been used.

This list is not exhaustive and combinations of the above points can also cause an oxygen deficiency.

Symptoms of oxygen deficiency

When there is a lack of oxygen in the pond, this can manifest itself as follows:

The fish mainly swim just below the water surface or at the outflow of the filter.
The fish have an accelerated breathing rate.
The fish are becoming lethargic.
The fish stop eating (there is insufficient oxygen for combustion).

Measures in case of oxygen deficiency

If the above symptoms occur, you may consider the following measures: