I've never heard anything good about the tablet CO2 systems.
I have had some luck with yeast-based DIY CO2 systems. It wasn't stable enough for my medium light 20L or 29g without having an army of 2L bottles, but a single bottle on either tank did keep BBA to a minimum and I was trimming the plants in the 20L weekly to keep some swimmable area in the tank.
The trick is getting the system to seal well. I built my systems as follows:
1. Tape a 20oz. Gatorade bottle to the side of a 2L bottle so that both bottles will rest on the floor when standing. The Gatorade bottle makes an excellent overflow bottle.
2. Measure the outside diameter of the airline you want to use. Bore three holes, one in the 2L cap, two in the Gatorade cap, that are slightly smaller than the OD of the airline. This will pinch the tube a little, creating a gas-tight seal.
3. Cut a short piece of tubing that will reach from the 2L cap to the Gatorade cap plus a few inches. Cut the ends of the tubing on a diagonal. Insert one end of the tubing into the 2L cap and the other into one of the holes on the Gatorade cap. Pull the tubing through the hole about 1/2" using a pair of pliers.
4. Run a longer piece of tubing from the tank to the other hole in the Gatorade cap using the same installation method as in step 3.
5. There are several diffuser options available. I used a piece of hard airline with a piece of cheap bamboo chopstick jammed in it. It worked reasonably well, but there are better options out there. You've now completed the hardware part of the system.
6. You can play with the mixture until you get one that suits you. I kept mine basic, but you can use different yeasts or different foods. I've even heard of someone using a jar of grape jelly with great success.
My mixture:
- 2 cups of white sugar
- 1/8-1/4 tsp of baker's yeast
- lukewarm water
It's simple, but I was able to get 2-3 good weeks out of it.
7. Mix the yeast with a few ounces of warm water to activate it. Hot water will kill the yeast, cold water won't do much. The water should turn milky and a little frothy as you stir.
8. Pour your sugar into the 2L bottle. A funnel helps a lot here. Add warm water to the bottle until it's two inches from the top. Shake the bottle until the sugar is dissolved. Put the top of the bottle in the palm of your hand to seal it while you shake, or use an undrilled cap.
9. Pour your yeast solution in the 2L. Shake some more to mix the yeast in.
10. Put your caps on the bottles. It helps to put the Gatorade cap on first, then twist the 2L cap counter-clockwise several turns to prevent twists in the tubing.
11. It may take a little bit, but at room temperature, I was usually seeing bubbles in the tank within an hour. If you don't, listen for fizzing sounds around the caps of the bottles.
One problem I found with my particular setup was that I was trying to inject the CO2 too deeply into the tank, especially on the 29g. The water pressure was preventing the CO2 from reaching the tank. I raised the diffuser up higher in the tank and I got a steady stream of bubbles.
I quit using it when I moved and I'm having BBA issues now. I'm looking for a good deal on a bottle so I can go pressurized. DIY CO2 just was too much work.
