Parts of a Bulkhead
A bulkhead is a plastic device that inserts into a hole that has been drilled in the aquarium to allow an external plumbing connection. The bulkhead makes the hole water proof and allows two connections for the plumbing -- A connection inside the aquarium and an external connection.
Bulkheads are either threaded or slip. Threaded means a device will screw into the bulkhead whereas a slip connection means a device will just slide into the bulkhead. If you purchase a threaded Durso Standpipe and you have a slip bulkhead, you will not be able to insert the standpipe. Be careful which type you order.
Typical Looking Bulkhead
The internal connection is where the Durso Standpipe will be attached. The external connection is for the drain pump to your sump.
Typically a "Reef Ready" aquarium implies that the tank has bulkheads.