TetraPond Bio-Active Pressure Filters with UV Water ClarifierBio-Active Pressure Filters force water through bio-activators which trap dirt and debris and perform biological filtration to help maintain clear and healthy pond water. It is designed with an easy, one-step, back-flush valve for cleaning. The unique bio-media design used in Tetra's Bio-Active Pressure Filters has a distinct open profile with a high active surface area. This allows unrestricted water flow to all the surfaces for improved cleaning during the back flush cycle. The open profile is designed to allow the force from the water unrestricted access to the entire internal surface area, rather than just the unidirectional flow that is provided by more common tubular shapes. Other media, such as closed bio-media (including foam), can clog up after extended periods of use, thereby reducing the active surface area within a bio-filter. Over time, old, dead bio-film can become virtually impossible to remove with normal backwash procedures and the bio-media may ultimately need to be replaced. In contrast, the Tetra Bio-Active Pressure Filter effectively exfoliates old and inactive bio-film from the inside of these bio-activators, leaving a young, thin and highly active bio-film layer for nitrification. That's the Bio-Active advantage. The Addition of a GreenFree UV Clarifier helps to remove heavy algae blooms usually within 1 to 2 weeks and keep your pond water clean, clear and green-free.