An air stone is a stone that is porous, that is lots of hole within the stone, abit like and aero choci bar and it has a piece of plastic at the end to attach to a pump. You would be putting on your Co2 though.
If you want to PM me your address i can send you one.
You would have to weigh it down once its in the tank.
Have a look at these.
http/www.aquatics-online.co.uk/Z058168.asp