Excuse me for intruding on your post Vanessa25, but I had a similar question; I'm glad you asked. I also appreciate the helpful advise that others have offered on this subject. I have a 20 gallon long with a 55W power compact bulb and my water is on the hard side (GH = ~8 degrees). I am experimenting with DIY Yeast CO2 (Nyberg method), as I plan to add plants that require CO2 supplementation. I am using one 2L bottle, but this does not seem to be producing enough CO2 (based on GH/pH table). After reading comments posted in this thread, I will probably add another bottle soon. My question is this: On average, what type of CO2 levels should I expect to obtain from the DIY yeast CO2 method? I understand that CO2 production using this method can be variable, but I'm just looking for a guideline to gauge how well my system is producing CO2. So maybe some of you experts that have long since moved on to compressed CO2 can think back to your beginner days and lend me some advice. Thanks in advance.