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.
Air
Breathing Fish
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
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.
Overcrowding
Perhaps the most obvious one is overcrowding. Naturally, if there are more fish, or bigger
fish, they use more Oxygen.
Over Feeding
When fish have eaten, their Oxygen requirement goes up
a lot. The food takes energy to digest. However, 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.
Decay
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.
Temperature
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.
Darkness
Plants can produce more Oxygen than they use if they
have enough light. At night they use it up.
Salt
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.
