All tanks have a threaded screw closure with gasket that secuely fits the 6 1/4" fill well. Two mounting flats near the bottom, set 180°> apart, permit installation of a bulkhead fitting up to 2". Domed bottom drains fluid into narrow trough around base rim. High-density polyethylene tanks have a 2" bung with with cap on the top that permits access for tubing or piping. All tanks are FDA compliant.