Aquarium Bulkhead Fittings are used to connect aquariums to sump style filters as well as overflow boxes. The bulkhead fitting utilizes a rubber gasket to make a watertight seal when properly tightened. (Never overtighten the fitting as you may risk cracking it and creating a leak) The rubber gasket always goes on the water side of the connection. There are many different types of bulkhead fittings. There are threaded and slip connections as well as many sizes from 1/2" to 2" being the most popular.