mystery chemistry - killing fish

[bwg]

changed water on my 55gal tank yesterday when after a couple hours the water became very cloudy and fish began to cling to surface

i did another water change before bed but to no avail

by morning my j dempsey was dead and platys are barely hanging on

can't figure out what the mystery in the chemistry is and i don't think it's the tap because it had no effect on another tank

 

[ScottoMacD]

A few of questions?

What kind of substrate are you using, and did you clean it?

Did you clean the filter?

Is there a possiblity of any sort of contamination?

 

[rus]

how large of a water change did you do and did you use declorinator?