TANKS & FILTRATION

Overflow Box

Gets water from an undrilled tank down to a sump without losing siphon on a power cut.

Setup stage Tanks

Unplug before servicingHands out of the water firstDrip loop on every cableTest before you dose30 ppm CO₂ ceilingCycle before stocking

Tanks

Specification
Type	Internal weir + external U-tube
Flow capacity	600–2500 L/h
Weir teeth	3–6 mm slots
Siphon	U-tube, self-priming on restart
Drain size	20–32 mm
Noise	Gurgle; silenced by a standpipe

Technical parameters
Weir water level	Sets display level
Drain capacity margin	≥ 1.5× pump flow
Sump volume on power cut	Must absorb drain-back
Restart prime	Automatic via U-tube
Air break	Required in return line
Noise mitigation	Durso or Stockman standpipe

A sump has to live below the tank, which means water has to get down to it. If the tank has no drilled bulkhead, an overflow box does the job: an internal weir collects surface water, and a U-tube or a pair of matched boxes carries it over the rim and down a drain pipe.

The critical design requirement is restart safety. When the return pump stops — a power cut, a tripped breaker — the siphon must either break cleanly without draining the tank, or hold and resume when power returns. A badly designed single-tube overflow will either flood the floor or lose its prime and leave the pump running dry.

The standard safe arrangement is a double-walled box: an inner weir inside the tank sets the water level, and an outer box behind it holds the U-tube. The outer box only ever holds a small volume, so if the siphon breaks, the tank drains no further than the weir height.

Safety and welfare

Size the sump to hold every drop that drains back when the pump stops, plus a safety margin. If it cannot, a power cut puts water on the floor.
Drill or fit a small air-break hole in the return line just below the waterline. It breaks the back-siphon when the pump stops.
Test the failure mode deliberately: switch the return pump off at the wall and watch what happens. Do this before the tank has livestock in it.

Using it well

A gurgling drain is a sign of air mixing with water in the pipe. A Durso standpipe or a slower drain fixes it.

Keep the weir teeth clear of floating plants. Duckweed blocks a weir within days and the return pump empties the last chamber.

Mark your normal sump water level with tape. A slow change in that level is the earliest sign of a clogged weir.

Works with

Sump systemCanister filter (as intake)Protein skimmerReturn pump

Note. Parameters are conventional ranges across the category, not the claim of any one manufacturer. Nothing on this page is for sale and no outcome for any plant or animal is guaranteed. Simulator output elsewhere on this site is a simplified model, not a measurement — confirm everything with a test kit and the equipment's own instructions.

Overflow Box

Using it well

Works with

Works alongside