Marine aquarium hobbyists whose tanks didn’t come with a built-in overflow or can’t be drilled to accommodate one (i.e., those made of tempered glass) sometimes choose to use a siphon-based hang-on-back (HOB) overflow to feed water from the aquarium to a sump. These overflows come in different designs, but they’re all more or less a variation on the same theme. In very simple terms, they consist of two chambers: a slotted box that is positioned inside the tank and skims water from the surface, and another box that hangs outside the tank. Water is drawn from the inside chamber to the external chamber via a siphon tube (or sometimes more than one tube). The water then flows under the influence of gravity out the bottom of the external chamber, through a hose, and down into the sump, where it’s pumped back to the main tank through a return hose. While this type of overflow can be a viable alternative to built-ins, it does have some possible drawbacks, not the least of which is the potential for flooding your fishroom in the event of a power outage, pump failure, or loss of siphon. More: How to Reduce the Risk of Flooding with Hang-on-Back Overflows… More:
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