2 teaspoons of yeast?? You're supposed to use like 1/4 to 1/2tsp.
If were getting 5-7 per SECOND, that is too much. But a few per MINUTE wouldn't be. I think you had too much co2 going in because you used too much yeast. The actions of the fish back that up. After using an airstone for a couple days, they should return to normal.
If you try again, only put 1/4tsp of yeast in each one. You shouldn't have the same results.
CO2 can be used safely to where the fish are not harmed.
Many, many, many people use it safely in their planted tanks without issues. I am not going to argue though, because some people have arguments for everything.
