Connect a check (one-way) valve to air line tubing 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, darnage 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.