Possible, but panda garra come from intermittent streams in the wild, and IME they are crazy adaptable for a wild-caught fish. Like you said, I'm not sure that applies to chlorine, but...I still suspect something else might have happened. I guess we'll never know for sure. I think it was Gary that pointed out somewhere how seldom we know exactly why a fish died. Sure is a bummer when chance strikes a favorite, though.
Sure, it could have been caused by any number of factors and I'll probably never know for sure but I still strongly lean toward chlorine as it just makes the most easy sense. I mean they died over night after a water change of ~40% and I had more chlorine conditioner the next day than I should have had or, at least, it seemed like I had too much conditioner left.
Fish DO die just as do all things but it bugs me when I don't exactly know why. Actually I've only had one tank that had zero fish deaths for over three or four years. That was a 55 gallon when I lived in Ohio that was all cichlids except for a pleco and a rope and used only under gravel filtration. Oddly the substrate was pebble gravel from a local fast running river and I very seldom did any water changes. That was a cool tank! When I get store bought gravel I'll boil to sterilize. With the river bed stuff I just rinsed well to remove sediment. Ended up with a killer tank and minute Crustaceans all over the place that the cichlids loved. The Crustaceans bred fast enough that they were always there. Probably the most healthy tank I've ever had and I did almost no real water changes except what it took to replace if I vacuumed the substrate. That tank also had a cichlid population that many would say can't happen. I had blue and yellow Acara, convicts, green terror and jack dempsey. The thing was that they all went in the tank as babies of no bigger than an inch so they just set up their territories early. I also made sure that they all had what the seemed to like. The rope had a slate cave system that I built to hide out in. The convicts also tend to like caves so I had suction cup fake caves mounted all over the back of the tank. It just all worked. LOL! My kid's friends would go out and catch crickets to feed my cichlids. Sigh, kids are such violent critters.
When I left Ohio in 1987 my kids took over the tank. When I left the tank had not experienced a single fish death.
I think that, sometimes, we tend to over think and over do our maintenance. In my specific recent case, if I had not done the water change, I would probably still have my Pandas.
Actually, with all I typed above, it is possible that I happened to just shock the tank's ecology. With dealing with a social security issue dealing with my benefits I kept putting off a real water change thinking I'll just do it tomorrow. The ~40% change I finally did MAY have caused a major shock to the tank's ecology. It kept going and was probably at least a month since I had done a real change. I probably should have done much smaller changes over several days. While I am not an advocate of major water changes in a larger tank my little 20 gallon cube does need them as it just isn't large enough to set up a self sustaining ecology.