The latest, the bottom drain with air dome. Deliver air to the deepest part of the pond to make sure there's no stratification. Additionally, and perhaps more importantly, the upsurge of air from the dome creates an impressive eddy which draws all settled solid waste nearby to the bottom drain. This bottom drain is ported for four inch with a barbed fitting. It's a "through the liner" bottom drain for serious hobbyists. You will need an air blower (we have these) suitable to apply air pressure to the bottom of the pond.