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A weir is a barrier, typically a panel of glass, that holds back a body of water. The water can then flow over the top of the barrier, with a constant water level then being maintained behind / above the weir.
They are most commonly used in marine aquariums to construct an overflow for the display tank (allowing surface skimming and constant water level) or to be used within a sump to separate components / equipment (particularly to provide an area which changes water level with evaporation for evaporation top off).
Examples of weirs within a sump, take note of the multiple water levels.
Simple tank with square corner weir.
- Overflow Capacity Calculator, Excel spread sheet used to calculate the capacity of overflows - reefs.org