Each tank in the main hatchery is drilled through the bottom of the tank with a 1 1/2 inch diamond type hole saw. A 3/4 inch bulkhead is used and a length of 3/4 inch PVC with a strainer on the top is used to drain away excess water. If I were to redo the system I would use 1 inch bulkheads and PVC instead of 3/4 inch. Another option is to drill through the backs or sides of the tanks, but that would have taken up extra room in my situation. The larger tanks outside of the main hatchery room are drilled in this manner as the bottoms are tempered glass and not drillable.
Water drains down through 3/4 inch PVC pipes into the sumps under the racks of tanks. The sumps are all interconnected via pairs of 2 inch bulkheads and PVC, and drain back to the filter system.