Non-return one way check valve for 4mm air line tubing. Connect a non-return one way check valve to air line tubing of the air pump to prevent water back-siphoning down your air line when there is a power outage. Back-siphoning is extremely dangerous and can ruin your air pump, empty your aquarium, damage your floors and create a shock hazard. Small and Compact Fits standard aquarium air line 4mm inner diameter Allows air to travel in one direction only. Stops water coming back down the air tube Easy to install – just cut the airline between the air stone and the air pump and insert check valve into air line