When fish hang at the water surface and breathe heavily, the cause may be a shortage of oxygen or poisoning. Many aquarium keepers and pond owners wrongly think that the fish are now gasping for breath. That is not correct. Instead, the fish are trying to reach the thin lay of water immediately beneath the water surface, because this water layer has the highest oxygen content. It isn't until the oxygen content is less than 5 mg/l, that an increase in breathing activity is found in most fish.

A remedy is quite simple: The oxygen content can be increased significantly within a few hours by using aeration pumps and air stones. The oxygen content can also be increased with the help of the backflow of water from the filter. As a general principle: the more the water surface is agitated, the more oxygen can be absorbed by the water. At the same time: the warmer the water, the less oxygen can be absorbed. Merely ascertaining why the oxygen content was so low proves difficult in this context. Dying organisms (fish, algae, plants) consume a large amount of oxygen when they are decomposed by bacteria. Algae treatment of all things often causes the oxygen content to decrease significantly, a situation that must be countered. Furthermore, it is imperative to check the nitrite levels with the test to check, since acute nitrite poisoning is recognised by the same symptoms.