I think I just killed all my fish

I cannot believe I did this but I think I managed to kill an entire tank of
cichlids tonight. I am really bombed out about it.

I have a 45 gallon tank with some juveniles thriving for the past 3 months.
Typically I changed the water once a month when I had some community fish in
there and once the cichlids were really small. Today I decided to test for
nitrates and switch to a biweekly water change. I had changed 25% two weeks
ago. The nitrates were kind high so I decided to a more aggressive water
change around 40% of the water, I used a python like I always do and filled
the tank up with water close to temp as the tank. The thermometer did not
register a temp change and the heater stayed up after it was full for maybe
3min I used a water treatment in the tank. immediately after this all my
fish went to shock, sitting at the bottom of the tank still gasping for air.
Couldn't understand what went wrong initially I thought they were just
frightened because of the substrate vacuum etc.

Two hours later still gasping and one already dead... I'm afraid they will
be more!

I am so mad right now.

Any comments as to what might have gone wrong are very much appreciated.

And just yesterday I was bragging about their colors and behavior to my
wife, I screwed my self so bad.

TIA

Steve