One bulkhead is plumbed to drain so it can be used to empty the tank by rotating the pipe down or as the primary drain if run on a flow through configuration. To make this work as a drain that can siphon the tank empty the plumbing needs to be vent free to below the tank. Just think of it as a siphon tube running through the wall of the tank. If you vent at the bulkhead you can run the intake on the bottom without emptying the tank. Generally I plumb the first way and if I want bottom suction I simply put a tee at the inside of the bulkhead instead of an elbow.
The other bulkhead is plumbed to a central filtration system in a surface skimming configuration. Water is supplied to the tanks via a plumbing manifold with a supply valve at each tank. Water can be from the recirculating system or through a hose for large water exchanges. All tanks have box or sponge filters.
