Step 1: Materials
A one liter gatorade bottle (or similar)
Several feet of airline tubing (vinyl is ok, but silicon is better)
An air diffuser
A suction cup to fit your tubing
A check valve
Aquarium grade silicon
A planted tank (duh?)
Step 2: STEP ONE
Step 3: STEP TWO
Step 4: STEP THREE
Step 5: STEP FOUR
About 6-10 inches away from the cap, cut the line in half, and plumb in your check valve.
MAKE SURE that the arrow is pointing AWAY from the bottle cap, otherwise you will have a ticking yeast bomb sitting under your aquarium.
The picture is of the finished project. Just pretend you don't see that part though ; )