From The Aquarium Wiki
A Check Valve is a simple device 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, airstone, etc. they negligent to fit a check valve.
When a 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 airstone then a siphon action can occur and the water ends up pouring out of tank!
The check valve only 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.
- Experienced aquarists know the benefit and security of always fitting a check valve. Accidents do happen!
There are two basic types. The slit and the spring.