I have an aquarium with live plants and I wanted to add CO2 injection so I made this yeast-powered CO2 reactor from a 2 liter pop bottle and some tubing.
Links to supplies (Amazon):
Airline Tubing – http://amzn.to/1eVh3ir
3/16″ Rigid Tubing – http://amzn.to/1KMJPvS
Check Valve – http://amzn.to/1KhijZk
Quick Set Epoxy – http://amzn.to/1ASKjR1
Airstone – http://amzn.to/1eVh7Pc