Pressurised Filters are those where the lid is watertight allowing the water to remain under pressure from the pump. This allows the filter to be situated below the waterline allowing it do be partially buried or placed below the pond or watercourse.

Most Pressurised filter have Ultra Violet Clarifiers (UVC) built in to prevent free swimming algae (green water) from proliferating maintaining clear water.

Pressurised filters usually combine both mechanical, to reduce particulates,  and biological filtration, to breakdown fish waste and keep the pond water safe for its inhabitants.