Mass tank deaths overnight

Ok so the tang was dead this moring. So one fish left now after this horrible ordeal and that is the firefish which I can't catch from the main tank.
 
...I will also do a water test to see if there is any trace of copper even though the solution is said not to contain it.

Keep in mind that copper will bind to the rock/sand, so it may be in the tank even if a test comes up negative, only to be released under changing tank conditions down the road (like a pH drop OrionN mentioned).
 
I will perform tests under different conditions also the treatment I used doesn't contain copper anyway apparently.
 
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