Overflow
The system that transports water from the aquarium to the sump by gravity. It can be internal (weir box) or external (hang on). Determines the maximum flow rate of the system.
Detailed explanation
The overflow is the ring that connects the aquarium to the sump. The water enters the overflow from the surface, descends by gravity into the sump where it is filtered, and the return pump sends it back to the aquarium. The cycle is continuous.
The internal overflows (weir boxes) are integrated into the aquarium. A glass panel forms a chamber in the rear corner of the aquarium. The water flows over the edge of the panel and down into the sump through pipes (typically 32 or 40mm). Brand aquariums (Red Sea, Waterbox, Cade) have internal overflows as standard.
External overflows (hang on, like Eshopps HOB or CPR CS) hook onto the edge of the aquarium. They use the siphon principle to bring water out of the aquarium. They are a solution for those who want to add a sump to a non-perforated aquarium.
The Durso system and the Herbie system are the two main methods for silencing the exhaust. The Durso uses a tube with a hole in the top that breaks the siphon and eliminates the sucking noise. The Herbie system uses two hoses: a full one (silent siphon) and an open emergency one. The Herbie is quieter than the Durso.
The Bean Animal is the three-pipe system: primary (full siphon, silent), secondary (open, safety), emergency (which drains in case of blockage of the first two). It is the safest and quietest system.
The sizing of the overflow must correspond to the flow rate of the return pump. If the overflow fails to drain enough water, the aquarium will overflow. If too much is drained, the sump empties and the pump runs dry.
In the event of an electrical blackout the water continues to drain into the siphon sump until the overflow is emptied. The sump must have sufficient volume to contain all the return water without overflowing.