These are the 2 CO2 bottles that I use for injecting the gas into my planted fishtank. They contain water, about a half cup of sugar, and bread yeast. I have to so I can alternate which one I change every weekend and get a more even flow of carbon dioxide into the tank. I have the little orange bread tag to tell me what one I changed last. Each bottle is good for about 2 weeks, but they peter out nearer the end, so it works out well to have both of them going. I used the juice bottles because I read somewhere that people have had their pumps suck the sugar water right out of soda bottles, collapsing the thinner plastic. Posted by Picasa.