I think I killed my fish during water change

[Crystal90]

No, definitely not used for cleaning. I store the bucket in my room. It's surprisingly not reached cold temps where I live. She was the only fish I lost so I don't think it was the last one.

I don't believe you did anything wrong. I've used seachem prime for years and you can't easily overdose with prime. I just use the bottle cap as a measure. I do 50% water changes weekly and unplug the heater but leave the filter running. I don't even check the water temp, just make sure the temp isn't overly hot or cold. I use a bucket that has only been used for the aquarium water changes. You didn't mention what breed of fish that died. Some breeds are not very hardy and a lot depends on where the fish is acquired from [meaning the fish store]. I've had a pleco that lived 23 years and other breeds that only lived months.