overflow

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The overflow box is used as a type of drain. Most pull gunk off the surface of the water.

As the water level (in the tank) rises to a certain level, the water enters the overflow and drains into (typically) a sump. The return pump sends the water back to the tank. With the overflow box, the water level in the tank maintains a steady level.

A Reef Ready tank has a built-in overflow box.

HTH

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