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Overflows and Return Lines

Why use an overflow box?

Some argue an overflow box provides for better surface skimming of DOCs (waste that collects at the surface) and better oxygenation and gas exchange due to the turbulence — like a waterfall.

The vertical location of the saltwater aquarium overflow determines your water level, so plan for wave-makers or any other potential “sloshing around” in the tank like maintenance and reworking your aquascape.  Higher water level might look better.  1″ is typical.

A surface level overflow vs. a typical internal overflow box has some benefits including
– NOT drilling holes in the bottom of the tank (max pressure) since all the water could leak if the hole or bulkhead leaked or failed
– doesn’t take as much space in the tank
– might provide a better overall appearance.

Note that a surface level overflow box might not be unstable if you have euro-bracing (which would cover access to the bulkhead).

Consider two drains for redundancy.  Consider the size of the drain(s) and return lines for total flow rate.  Note that the physical size of the holes is generally bigger than the bulkhead size.  For example, for 1 1/2″ bulkhead, drill 2 3/8″ hole.

The Xaqua surface level overflow system with oscilating return line

The overflow in the Oceanic tank draws water from low, mid and high in tank improving overall flow. Look closely for the small, inside compartment.

DIY approach with return line to the left of the drain.

note drain line made of corrugated drain hose

the pvc elbow could be rotated to fine tune water level; cover it with some sort of screen to prevent an animal from being sucked in


Calfo Overflow — wide overflow box with redundant drain lines