The reservoir tub is a very popular way to add extra volume to a Seamless Sump® configuration.
Drilled for Reservoir / Refugium
If you choose to have the tub drilled as a reservoir or refugium, a 1/2″ bulkhead is placed 11.81″ from the bottom of the tub. This gives you a nice 16.6 gallon refugium for fish, coral, or plants. This is very common and recommended for saltwater aquariums.
Drilled for Evaporation
As an evaporation tub the 1/2″ bulkhead is placed 2.31″ from the bottom of the tub. This provides you with a little more than 10 gallons of evaporation before the water will fall below the recommended level for most standard pumps. This is a most common with freshwater aquariums.
If you have room in your stand it is ideal to have two reservoir tubs. One as an evaporation tub and one as a refugium.