Oxygen and Fish
Fish need Oxygen. So do humans and most of the living things we are familiar with. We breathe air, but most fish cannot do this directly, and need to get theirs from the water. The water molecule is H2O, but the “O” atom combined in the water molecule cannot be used by fish. The Oxygen that most fish use is dissolved in the water.
A number of groups of fish have ways of using the Oxygen of the air as well as that dissolved in the water.
The Labyrinth Fish (anabantids) including the Siamese Fighting Fish, the Paradise Fish and the Gouramis can absorb it from the atmospheric air through their labyrinth organ. These fish frequently live in still water which can have enough decaying vegetation in it to remove a lot of Oxygen.
Another group of air breathing fish familiar to most aquarists is the Corydoras catfish like the Bronze Catfish. These fish absorb Oxygen through their intestine. They can be seen making a quick dart to the surface to take a gulp of air.
An obvious group of fish that breathe air is the lungfish like the Australian Lungfish.
There are other fish that are adapted to use Oxygen from the air including the Mud Skipper and the Climbing Perch. These two fish actually come onto land at times.
Most fish have to use Oxygen that is dissolved in the water. Normally the fish that come from well Oxygenated water, like fast flowing streams will need more Oxygen.
Oxygen in Aquariums
There are several things that can reduce the amount available in aquariums.
Perhaps the most obvious one is overcrowding. Naturally, if there are more fish, or bigger fish, they use more Oxygen.
When fish have eaten, their Oxygen requirement goes up a lot. The food takes energy to digest but a bigger effect occurs if the fish do not eat all the food. In this case the decay of food removes Oxygen from the water.
If anything decays in the water it will remove Oxygen. This applies whether it is uneaten food, a dead fish, or a dead or dying plant.
As the water temperature increases the amount of dissolved gasses it can hold drops, while at the same time the metabolic rate of the fish rises, so they need more Oxygen.
Plants can produce more Oxygen than they use if they have enough light. At night, they use it up.
Adding salt to the aquarium water reduces the ability of the water to dissolve gasses.
Putting Oxygen into the Water
Bubbling air through the water from an air pump will help to oxygenate the water. The effect of this is two fold. Some air is dissolved directly from the bubbles, but another major effect is to move the water, particularly bringing Oxygen deficient water from the bottom of the tank to the surface where it can dissolve Oxygen.
This increased dissolving of Oxygen at the surface is also enabled by a power filter that circulates the water, including bringing bottom water to the top.
Green plants produce Oxygen while it is light. Faster growing plants produce more than slower growing ones. If the plant is floating, most of the O2 produced goes into the air. It is only the submerged plants that add much to the water.