Air check valve

[LinY]

A very, very basic question: I recently purchased an air pump and installed an air check valve. Of course there is nothing which says which direction the valve is installed in the tubing. (Actually, the one I purchased was apparently defective--no air flow either way it was installed--and so the company is sending another). So, when the new one comes, which way?

[madagascariensis]

try blowing into it from both ends(not at once, alternately).
it should be pretty obvious which way air is allowed to pass.
make sure that there is nothing else that could possibly be hindering air flow, such as a malfunctioning pump, a clogged airstone, or one placed too deep in the tank.

[raven]

most of them do have an arrow on them pointing wich way