When I started up my 30 gallon tank, I set up a 2 liter bottle with the sugar/yeast mix. The glass difusor was situated right under the intake of my filter uptake so most of the bubbles went into the filter. I bought 9 neon tetras, and the next morning, multiple were dead. Actually, they started acting funny within hours of being introduced into the tank. After another day the rest were dead. They swam really funny like thy were drunk. I'm convinced it's the DIY because the ph and other levels tested fine. I yanked out the system, and the fish I've added since are doing well. How do you add CO2 without killing fish??? For now I'm sticking with my excel which many people rave about.