Bacteria in your DIY CO2? Try this:
I knew the stench of bacteria the instant I smelled it. IMO, there's no point in trying to use proper aseptic technique in the kitchen to keep bacteria out of the brew 'cause it just ain't going to happen. Instead, I add an antibiotic that everyone has in their house. Aspirin. It's fine for yeast, but it slows down some bacteria and kills others. I had no clue whether it would work for whichever bacteria species take over yeast cultures, but I tried it, and it works. My yeast are cooking away, and the stench has never returned.
I use three to six 325 mg tablets per two liter bottle.