There was no need for panic. The fish were not clustered at the top of the tank, so they were still able to tolerate even the high level of CO2 that you had. As soon as you shut off the CO2 that level began dropping, and within a couple of hours it would be low enough to be no problem at all. Changing water isn't necessary, letting the CO2 outgas from the water was necessary, but you did that.
Cheap regulators let lots of people, who otherwise couldn't afford it, use pressurized CO2. But, they need to learn to live with the quirks of those cheap regulators. One of the things to learn is to check the CO2 system every day, looking at the inlet pressure to see if it is dropping. Once it drops below about 500 psi, either adjust the output pressure a few times during the day, or just refill the CO2 bottle. And, of course, checking the bubble rate every day is a good idea too.