The diffuser replenishes the oxygen present in the aquarium and enriches the aquarium water with oxygen. The oxygen supply is recommended for aquariums with a small population or those with a large fish population. Also for cold water and salt water aquariums or for the maintenance of oxygen hungry fish. Also for the case where a cover limits the air space above the water surface, additional air should be provided. Otherwise the oxygen in this area would be quickly depleted. Tip: EHEIM VARILUX Covers (VARILUX Aquarienabdeckungen) guarantee permanent air circulation thanks to caf (constant air flow) technology.