I read this in an article
For around a 30 gallon tank.
one 2 liter bottle.
2 cups of sugar
1/4 tsp of yeast "Fleischmann's active dry yeast" I use.You can use what you want.
You need to drill or punch a hole in the cap, then insert some airline tubing so about its an inch *inside the cap. Use a sealant to seal the air tubing in place. Let it set overnight to be safe. Rinse it lightly the next day.
Have the yeast come to Room temp first.Then add the sugar and yeast into the bottle.Fill the bottle up with warm water(not hot) half way. Shake up. Fill the bottle about 4 inches from the top.
For best results in my expierence, use a bucket or a small tank with a small heater.Fill the tank or bucket so that the DIY Co2 bottle is in a heated bath.If your house is in the 70's you might not need to do this.
I put mine below my tank with a check valve inline.
So far at 78 f, I get a bubble a second or every two.
Make sure your yeast is not older than 6 months.
Make sure you refrigerate the yeast or the yeast has been refrigerated(if open)
I found yeast in a seeled jar (with the air locked saftey cap) is best. Other yeast was not active for me(from a farmers market)
You can hook up two bottles with a connecter for bigger tanks.