The Air Stone (4 inch) (normally used in aquariums)
The easiest way to provide the needed oxygen for the tempeh
Keep the air stone wet at all times. 
The Air Bubbler Tube, if an air stone can not be found:
The purpose of this kind of tube is to pre-warm and pre-moisten the incoming air. The air enters the bottom of the tube where we glued the tubing into the plug with some aquarium cement to keep it in place. 
Notice the placement of the tube in the sketch of the incubator - it goes from low to high. As the air rises to the higher exit point it moves some of the water along with it and this actually helps the water circulation in the cooler.
