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All plants and animals respire one way or another, respiration uses oxygen to release chemical energy from a food source for use in the body. In an aquarium, the more living organisms are present, the more respiration takes place, and the more oxygen is used up. When excessive levels of waste are present, or an aquarium is densely packed with plants and there is little surface movement, high levels of respiration can cause oxygen levels to become dangerously low.

In most aquariums, the biggest user of oxygen is bacteria, which is why the filter outlet is often placed at or above the waters surface, where the returned water can be re-oxygenated from the surrounding air.

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