So small catastrophe 2 weeks ago. I just got a new dual regulator for my Co2 cylinder and ran it for a day. nice light green drop checker. When switching out the regulators, I noticed the cylinder was low and took and got it filled.
I hooked it back up and left for the afternoon. When the Co2 came on, apparently either the needle valve moved, or the cylinder was lower than I thought and the pressure went up because I came back around 10pm to all 23 fish floating at the top or upside down at the bottom. OH CRAP!, gassed them!!, pulled the the outflow up out of the water and did a massive WC as quick as I could. I left the smaller fish to fend for themselves and revive, but I held my three angels in my hands below the oxygenated outflow current to help revive them for 30 minutes.
My Koi female came out of it, but the two males did not survive. I lost one blue tetra as well, but 12 cardinals, 3 blue tetra, 1 powder blue gourami, 2 plecos, and three corys came out of it alive. Also, my two experimental RCS suprisingly also are still running around.
since then I have purchased a new bubble counter that sits on the regulator and moved the drop checker to the middle of the flow from the diffuser instead of at the end, so I can know sooner if I have an issue. Since then the Co2 has been very steady, but learned my lesson at the expense of my two beautiful Angels. Live and learn!