@Thread title. Please spot what went wrong as we too are a bit miffed with the unfortunate things that happened with our critters. (2 silver dollars and 1 oscar have died the other one is clinging)

Seriously, i feel so bad coz they were growing so nice.
So here goes:
My father wanted to have a hand into cleaning my younger brothers' aquarium (a 50 galloon one), for the most part my younger brother has been cleaning the aquarium via water changes and this time we weren't able to budge in the type of cleaning my father wanted so we ended up just letting him. He removed everything, put the fishes on a bucket using tank water and proceeded into cleaning the aquarium with some soap. My younger brother cleaned the sponge filter that it had with tank water. Almost an hour or so passed and i was stunned to hear that 2 of our silver dollars who were separated has died. I think it has to do with the water quality.
Nevertheless my father and my younger brother rushed to put things back together and after some issues with the power head they were able to after another 2 hours. However 30 minutes after the fishes have been placed back in the "cleaned" aquarium one of the oscars started to show signs and a couple of minutes later it passed away. We were totally miffed, i on my part suspected that we unknowingly pushed the reset button. (i.e. hence the question-ish thread title)
About an hour and half later the other much larger oscar showed signs and now it's clinging to dear life.

I totally feel devastated as of the moment even though it's not my tank, i mean we have seen this clinging oscar grow to it's current (and probably final) 4-5 inches.
Did we unknowingly cycle the aquarium? Or did the fishes that expired died from some other reason? (i.e. the water quality of the tank water bucket that we temporarily placed them in?).
Feeling so sad right about now.
