I just finished up my DIY co2 system for my now planted goldfish tank. The tank is a 20gal L tank. I'll draw up some diagrams after work to post here as well. But for now enjoy these pictures.
The basics of what I did was take a cheap powerhead, $19 at wally world, an aquaclear 70 foam filter insert, some fluval biomax rings, assorted pieces of PVC pipe and vinyl tubing. All of which is hooked up to a 2 liter soda bottle generating CO2 through good old fermentation. But, i put an airstone hooked up to my CO2 generator towards the top of the tube, which is attached to my generator. In the elbow joint is a drain cover which keeps the media in place and also holds the airline tubing. Then i filled the tube with the biomax cylinders, attached the elbow joint at the bottom, pulled the airline taught, attached it to my generator, Jammed the sponge on the powerhead, and flipped the thing on. It works really well and is really really quite. Instead of a steady stream of CO2 bubbles coming out of my generator and rising to the surface to just be wasted, I've now got nice, filtered, water that is CO2 rich.
CO2 System