Not remembering if you added dechlorinator....my question is kind of moot.
I would ask if you overdosed Prime. I killed half a tank of fish doing this. I was stupidly treating RODI water with an excess amount of Prime, so there was nothing for the Prime to bind to except trace levels of nitrates and then...oxygen. Finished the water change at 5 pm, I came in to check on my tank at 3 am, half my fish were dead with the other half on their way out. Looked at my drop checker, I had never seen that thing so yellow(CO2 is on a timer, shuts off 1.5 hours before lights out at 5 pm, yes it was off). Did a quick 50% water change, threw in an air stone, by the next day everyone that survived was right as rain.