This is a discussion on Did Co2 Do This? within the Tropical Fish Diseases forums, part of the Freshwater Fish and Aquariums category; -->
Well here is an area of expertise for me! I'm a firefighter and co2 can indeed be deadly to humans in high enough concentrations. ...
Well here is an area of expertise for me! I'm a firefighter and co2 can indeed be deadly to humans in high enough concentrations. However unless the room was entirely airtight (and trust me I've yet to ever see one) and maybe not even then, theres no way a small co2 reactor in the form of a 2liter bottle with some yeast could build up the co2 levels high enough to cause a health threat. If on the off chance you were to walk into such a room and begin to feel dizzy or sleepy I'd suggest exiting immediately and finding some means of ventilating the room. Realistically though this should not be a problem at this scale and with general home construction.
understood and indeed better safe than sorry and its not as if you were totally off base. But assuming he's not actively trying to make his room airtight it should be no problem whatsoever. If we were talking carbon monoxide it would be an entirely different story but it takes a whole lot more carbon dioxide at a much higher concentration to cause serious harm to a person.
With that much foam I'd say your yeast mixture got into your tank. Smell it, does it smell like what's in your 2L bottle? If it does, you get to do a water change to try and dilute it enough. The yeast shouldn't hurt your fish I wouldn't think, but it may cause an algae bloom.
And there's no way a 2L bottle will produce enough CO2 to be harmful to humans, or small animals. I've had 15 gallons of beer brewing at once in my house, and I'm pretty sure I'm still ok. :D