Oxygen 

All living organisms require oxygen to breathe. Fish extract oxygen from the water via their gills, if there is insufficient oxygen present the health of the fish will suffer. Oxygen enters the water by diffusion at the water surface, this is occasionally referred to as the air/water interface. Oxygen passes into the water as the surface is agitated, the more agitation the more oxygen introduced.

Fish, plants and bacteria all use up oxygen and as a result supplies can become depleted, especially at night when plants stop photosynthesising but continue to respire. The amount of oxygen needed is dependant upon the type of fish you keep. Some fish require no additional oxygen as they are able to breathe atmospheric air. 

It is possible to have too much oxygen present in the aquarium, this is dangerous for fish. Water has a saturation level which is the maximum amount of dissolved oxygen that the water can hold. The saturation level alters according to the temperature of the water. Heavy photosynthesis in densely planted aquariums can lead to the water becoming 'supersaturated'. If your water is supersaturated with oxygen the blood supply of the fish will also become supersaturated. This can prove fatal if not corrected. Ensure that you have adequate aeration as this will help to exchange the gas at the air/water interface (oxygen will be released from the water as well as absorbed). 

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