Man, that stinks! Sorry to hear that. Did you not have access to a big bottle of dechlor? Chlorine is neutralized very rapidly. Always handy to have a bottle or two of the stuff around.
I once didn't add enough dechlor to a tank when doing a large water change. The chlorine levels were low enough that any effects weren't immediately seen, but my wife stumbled across the tank a couple hours later after nearly half the fish were dead. After I rammed my head into the brick wall, I threw in a bunch of amquel and saved the rest.
Sounds like your chubs were building a nest out of the only materials they had available. Pretty cool. Time to start again.