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They are an overflow that hangs on the side of the tank, which does not have to be drilled. The siphon box is essentially two boxes, one inside the tank and one outside that are connected by a siphon. Care must be taken to ensure bubbles do not get trapped in the U tube that connects both sides, otherwise the siphon will break. These can have problems associated with the siphon not restarting after a power failure. Some designs include a suction line on the top of the siphon that can be connected to a small pump to ensure the siphon is always maintained.