Bottom drain options: The standard drain is available with or without an air diffuser. (This unit is just the 3" bottom drain only.) Each style has two rubber gaskets and six stainless steel screws for a water tight penetration through the liner. For use with 3" PVC pipe and fittings. Bottom drain with air diffuser has 1/2" barbed air inlet, recommended air flow of .5 to 1.5 CFM. The heavy duty bottom drain is designed for use with 4" PVC pipe for high flow applications. This bottom drain is roto molded for superior strength and durability! Unlike other bottom drains which have multiple legs around the outside (which debris can catch on) this drain has a center leg which prevents plugging. The large dome top will keep fish and large debris from entering pipe. Price includes drain and fittings to connect to 4" pipe. A 4" x 3" adapter can be used for 3" pipe.