You really want the check valve going the other way. So the water can't come back into the co2 bottle. If you are paranoid about yeast spill-over into the tank the best thing you could do would be to put a gas seperator on the setup. With a gas seperator (usually a 20 oz soda bottle) if you do get an overrun of yeast it would use that as a spill-over instead of your aquarium. This also lets you use the check valve the other way to prevent water from siphoning back.C_perugiae said:After helping a guy whose CO2 bottle was knocked over by his cat under the stand... yes, I'd be adding a check valve; the yeast backed up into his tank and the water was opaque by the time he got home from work.