For aquarium inhabitants, it is extremely important to have a uniform water temperature that is appropriate for the species, and that is consistent. The fewer the fluctuations the better. Temperature fluctuations means stress and a possibly life-threatening situation for fish and other aquarium inhabitants. If the water temperature increases, the metabolic activity of the fish also increases considerably. The oxygen content of the water is also temperature-dependent. The warmer the water, the less oxygen it can absorb.