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Liquid water (H₂O) is a medium in which fishes and other aquatic organisms have adapted to live. All natural water in oceans, lakes, ponds, puddles, rivers and brooks has varying amounts of other substances (either dissolved or mixed) in the water. Groundwater, tapwater and moisture in the air is also more than just pure water.

Fishes usually live submerged in water and constantly extract dissolved oxygen gas from the water to live. There are some exceptions, because some fishes can also extract oxygen from the air, like for example lungfishes, arapaimas, snakeheads, gars, climbing gourami (and other labyrinth fishes) and some species of catfishes and some species of loaches. They have evolved organs and developed methods to use athmospheric oxygen to supplement (or replace) the gills function, depending on the situation. A few of these exceptions have also developed methods to travel on land, or bury themselves in a cocoon in the mud. However, for the vast majority of aquarium fishes it's very important that the water in the aquarium always has an appropriate level of dissolved oxygen gas, within a tolerabele range for each species.

With higher temperature, less amount of gases and hence less oxygen gas can stay dissolved in a water solution. At the same time, the metabolism of fishes, bacteria and other organisms (except warm-blooded organisms), increase in warmer water compared to cool water and hence their need for oxygen gas increase in warmer water.

The atmosphere on our planet Earth has varying amount of water vapor or water moisture in it, but if you remove the water from this air you get dry air. Counted by mole fraction, this dry air usually consists of about:

~ 78.08% nitrogen gas (N₂)

~ 20.95% oxygen gas (O₂)

~   0.93% argon gas (Ar)

~   0.04% carbon dioxide gas (CO₂)

~   Trace amounts of various other gases.

However, the amount and types of gases may vary a lot in enclosed spaces. It also varies near combustion (fires, engines etc.) and near animals (caused by respiration, farting, defacating, urinating, sweating and other activities) and near decomposing matter and near geothermal activity like volcanos, sulphur pits etc.

Similar to the distribution of gases in air, gases are also dissolved and/or mixed in water, since the athmospheric pressure naturally forces gases into the water. Other sources of pressure, such as mechanical pressure, moving water pressure, or water pressure at different depts can also compact gases in the water. It is possible to remove most of the gases from water if you apply vacuum to the water that will suck out the gases.

When fishes from the deep rises up towards the surface, and if the fish has a swim bladder, the swim bladder can get damaged, during (and after) rapid ascension, as the gases inside the swim bladder expands. The swim bladder may even burst and even if the swim bladder does not burst, the surrounding organs and tissue may still get damaged from getting squashed by the swim bladder as it expands. Gases inside the bloodstream and cells may also start to expand, and may form dangerous bubbles, when the surrounding water pressure rapidly drops. That is why many wild caught fishes that live in deep water usually need time to adjust when brought up. They should be brought up slowly, perhaps during a period of several days, to avoid harming the fishes. An alternative, but probably more expensive solution, is to use a pressurized container, or a pressure chamber, to bring them up, and to slowly adjust the pressure in the container or chamber. In an emergeny, or during time constraints, a hypodermic needle can also be used to release som of the gas from the swim bladder, but this may also damage the fish, so it is prefered to avoid this procedure, if it is practically possible to solve the sitation in a less damaging way. The gas composition of the gases inside a swim bladder varies in different types and species of fishes. It is common among fishes to have mostly oxygen gas inside the swim bladder, but some types of fishes instead have mostly nitrogen gas, while others types of fishes mostly have a mix of oxygen gas and carbon dioxide.

Plants and algae produce oxygen gas (O₂) from photosynthesis using carbon dioxide gas (CO₂) during the day when there is light, but at at night, when there is darkness, the plants and algae instead consume oxygen gas and produce carbon dioxide./P>

Carbon dioxide gas (CO₂) and water (H₂O) may turn into: carbonic acid (H₂CO₃)

Carbonic acid (H₂CO₃) may also turn back into: Carbon dioxide gas (CO₂) and water (H₂O)

Most benifical bacteria are in need of available oxygen gas (O₂) to efficiently perform some steps in the nitrogen cycle. Bacteria usually use oxygen gas (O₂) to break down:

Ammonia (NH₃) or ammonium (NH₄⁺) to Nitrite (NO₂^-)

Nitrite (NO₂^-) to nitrate (NO₃^-)

Oxygen gas is also used by bacteria to break down many other substances in the water.

To ensure that there will be a sufficient oxygen level, you can use an airpump to airate the water, but usually this is not neccesary if you have a normal amount of livestock and no species that demand a very high oxygen level. A waterpump that makes a current on the surface of the water will usually do fine. However, an airpump might still be useful, especially if you have a lid on the aquarium that partially or totally inhibits the exchange between the air in the room and the air trapped in the space under the lid above the aquarium water.

An other way to increase oxidation is to use small amounts of hydrogen peroxide. This may be used as an alternative, or complement, to biological filtration, since hydrogen peroxide is very reactive. An oxydator is a contraption that is made to slowly release small amounts of hydrogen peroxide in water. It is also possible to use electrolysis to create oxygen gas, and small amounts of hydrogen peroxide (and hydrogen gas as a byproduct) directly from water.

An other way is to add pure (or diluted) oxygen gas to increase oxygen. This is often used instead of air, during shipping of aquarium fishes, in plastic bags. However, be careful whenever you pack fishes that can also extract oxygen from the air, since they should not be packed with pure oxygen gas. This is because it may damage them. When shipping fishes that can also extract oxygen from the air, it is better to use normal air, or air that is only slightly supplemented with oxygen gas.

In a heavily planted tank it is possible to increase oxygen concentration by adding carbon dioxide during the day, since the plants will convert/consume carbon dioxide and produce oxygen gas during photosyntesis.

The pH-value and hardness of the water is often important when you are going to breed fishes. It's because the fishes and their eggs have adapted to certain water parameters in the wild. If these values are too low or to high compared to their natural habitat in the wild, the fishes may feel bad or don't feel very enticed to spawn. If they spawn anyway, maybe the eggs can't develop properly in those unhospitable water parameters, and if so, then the eggs may be ruined or have a low chance of hatching.

The temperature should in general be around 24 to 26°C for most normal tropical fishes. Species that live in for example fast flowing mountainbrooks may prefer or need a bit cooler water. Species that live in for example calm ponds that are heated by the sun, or geothermic energy, may prefer or need a bit hotter water.

When it comes to water changes many people think that equals to change all the water in the aquarium. However, it's usually not so good to do it that way. It will hurt the balance in the biological system that has (hopefully) had time to settle. Instead, if you change about 30% of the water regulary, about one to four times per month, both the fishes and the plants will probably feel better.

It may perhaps be even better if you do partial water changes more often, if you have time to do so, or if the aquarium is heavily stocked, but it should preferably be done on a regular basis. Fry and some sensitive species (like discus) may need larger water changes, more often, to grow well and prosper. On the other hand, if you have a very sparsely populated tank with only hardy fishes, but lots of plants (or even an aquaponics system), there may perhaps not be any urgent need to do water changes, but it is still recommended to do water changes once in a while.