A Bulkhead Fitting / Tank Adapter is a special plastic fitting to provide a watertight seal through a plastic container or a flexible pond liner. This fitting allows full flow of water and the ability to attach flexible pipe with the use of a threaded male adapter. Bulkheads work best on tanks with flat walls or with only a slight curve.
Uniseals are the best alternative to bulkheads for tanks or pails with curved walls. They proved a watertight seal even on round pails. No other fittings are needed, just drill the correct size hole in the tank or pail, insert the Uniseal, and then insert the pipe.