Phew, that was a close one!
I had decent CO2 levels in the water already when I got this wild hair up my arse that it would be a good idea to try and kill all those little bugs on the water surface by suffocating them with CO2. I didn't realize that my water CO2 levels were already near the limit 'cause I don't yet have a drop checker or even know what the hardness is. So I go taping a plastic sheet over the top of the aquarium and pumping CO2 gas under there to fill it like a balloon. 5min later all three of my gorami's are unconscious.
I did manage to save the gauramis by scooping them into another, non-planted, well oxygenated tank. They began to perk up right away. I had to hold the one by the tail so his mouth was over the air bubbler. I thought for sure I was going to lose him but he made it.
SCARY! Next time I try that stunt it will be in the morning when the water has out-gassed all it's CO2.
There was no warning though. No fish scooping at the surface or breathing heavy. Just one minute fine and the next almost dead. Apparently the Gauramis are the most sensitive fish in the tank though. The rest of the fish weren't struggling so much.