Aquarium Central Air line Check Valve are simple devices used in airline tubing to prevent water running backwards down a air tube and flooding the room. Often when beginners buy a air pump, tubing, air stone, etc. they negligent to fit a check valve. When an air pump is operating normally the pressure of the air pushes the water out of the tube. However if the tubing became detached from the air pump end or the air pump stopped working, what can happen is that the air is forced out of the tubing by water pressure and if the end of the tube connected to the pump is below the level of the air stone then a siphon action can occur and the water ends up pouring out of tank! The check valve allows air to flow in the one direction and therefore prevents water getting too far down the tube. You cut and fit the valve near the top of the aquarium.