Fish need oxygen to breathe, just as humans do. Since they don't have lungs, however, they obtain their oxygen from the water they inhabit. The air we breathe contains 21% oxygen. However, water only has a very small amount of dissolved oxygen (oxygen that is dissolved in the water). Fish use their gills to move oxygen into their bloodstream in the same way that humans use lungs to take oxygen from the air. If fish do not have enough oxygen, they can suffocate and die. This is why it's important to take proper steps to ensure your aquarium has enough dissolved oxygen. Luckily, taking a few simple measures can achieve this.
The amount of oxygen water can hold is called saturation. Water temperature, salinity, and atmospheric pressure all affect the level of saturation. A number of factors can contribute to low dissolved oxygen in an aquarium:
The most familiar step is to install an air stone or bubble wand. While most aquarium hobbyists are already familiar with these, I have seem claims that they aren't necessary for a thriving tank environment. While a few fish species can tolerate a lower level of oxygen, they would still benefit from having a good amount of dissolved oxygen available to them. Of course, whether using a stone or a bubble wand, they are powered by air pumps and connected with airline tubing.
Along with pumping extra oxygen into the aquarium water, the aeration system mentioned above will help circulate the water and break the surface tension. The rate of diffusion increases by doing this. Placing an airstone near the bottom adds oxygen more efficiently than having bubbles occur at the top of the tank since oxygen exchange occurs at the water's surface. Stagnant water has lower oxygen levels and isn't able to reach the lower level of the tank.
Powerheads, water pumps, and filters also play an important role in circulating aquarium water. You can also increase water movement by adding an additional filter or replacing the current filter with one of a higher gallon per hour capacity.
Adding aquatic plants can also help to increase the level of oxygen in the water because of the photosynthesis process. A large benefit of live plants is that they produce oxygen and absorb carbon dioxide and ammonia. Though it is uncommon, live plants can contribute to low dissolved oxygen if there isn't enough light provided. When exposed to light, plants produce oxygen. When there is no light, however, all plants consume oxygen. If a tank has no or low light for a long period of time, oxygen can become depleted. It is generally recommended to have light on a planted aquarium for 8-12 hours a day. The type and intensity of lighting will depend on the size of the tank, and the specific requirements of the individual plants.
Avoiding overstocking the tank will also help. All fish will take in oxygen, so having too many fish consuming the available oxygen will, of course, shorten the supply. Most of the time, it is recommended to have no more than 1 inch of fish per gallon of water. (Goldfish are an exception to this rule - for goldfish, 5 gallon per 1 inches of fish is the typical suggestion).
It also can't be stressed enough how important it is to maintain good water quality. Excess waste can lead to spikes in ammonia and nitrates, as well as the depletion of oxygen, so regularly performing water changes and cleaning substrate will help to retain oxygen in the water. Keep in mind, too, that higher water temperatures don't hold as much oxygen as lower water temperatures.
If the oxygen levels in your aquarium drop too low, it will affect the behavior of your fish. Early warning signs of this are fish being less active and possibly eating less. As levels of oxygen decrease further, fish will display labored breathing and faster gill movement in an effort to get sufficient oxygen from passing more water over their gills. Finally, they will begin gasping at the water's surface, sometimes opening their mouths very wide.
If you observe fish gasping, emergency steps should be taken immediately to increase the oxygen levels. The first step you should take is to perform a large water change. At this time, you can also add a filter, air stone, or powerhead if you need to increase water flow.
Once good oxygen levels are reestablished, any corrective steps should be implemented and continued to keep the fish healthy and happy.