Aquarium pump is used to supply air to your aquarium. May have one, two or more outlets. Those outlets are connected to a diffuser placed at the bottom of the tank, which is making small air bubbles nicely rising from the bottom of the aquarium. Pump can be also connected to the internal filter. Those bubbles also increase the water circulation. Water is usually warmer at the top so it helps to mix it with colder water at the bottom.
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