It is obvious that oxygen is needed for something to carry on living. Just like everything on land, everything in water also needs oxygen to survive. Saltwater aquariums generally need more oxygen, because of the warmer temperatures. If the oxygen level decreases, it can lead to suffocation of the fish and the result being dead live stock.
When your fish are looking sick, swim slower than usual, or gather at the top of the tank, that means that the oxygen levels in your tank are too low and you should stabilise it as soon as possible. Low levels of oxygen cause negative bacteria to build up in your tank and may harm your fish. Look out for changes in fish skin colours, because that is normally a sign of disease.
It is important that you have a system that is powerful enough to circulate the water in your system, including the water at the bottom, so that oxygen levels are maintained in your tank. Water also needs ventilation, so make sure that air is able to enter the top of your tank. It also helps to not overstock the tank, because with too many fish polluting the water and competing for the same air supply, can cause low oxygen levels. Another cause of low oxygen can be a lack of surface area. Oxygen is also needed for photosynthesis to take place. If you do not have enough oxygen, the plants in your tank will not be able to photosynthesise and your circulation system will have to work twice as hard to put oxygen back into the water.
There are certain fish that are able to live in a low oxygen environment, for example Bettas and other Anabantoids, because they breathe directly from the surface in order to receive most of their oxygen. Although these fish can live in a low oxygen environment, it is still not recommended to have low levels of oxygen in your tank. Always check your oxygen levels and make sure that it is completely stable. Fish can live up to two weeks in a low oxygen environment, depending on how much fish there are in the tank. If they are fairly crowded, they might not make it past two days.
You can also have too much oxygen in your tank. Too much oxygen in water can lead to the potentially lethal gas bubble disease, in which gas comes out of solution inside the fish, creating bubbles in its skin and around its eyes.
Air stones can be used to prevent this. An air stone is a piece of aquarium furniture that gradually defuses air into the tank, eliminating the noise and large bubbles of conventional air filtration systems. It will help increase the water circulation more than an undiffuse air feed into the tank from an air pump, and the bubbles themselves will also increase the surface area of the water more, but the real benefit from an air stone as far as oxygen goes is just the increase in circulation.