OK sounds like something that happend to me though.
Instead then I want you to take careful looks at your heater and power head, make sure that the heater is functioning correctly and has no condensation inside of it if it is a glass one, make sure to inspect the entire length of the electrical cords to make sure that there are no breaks or open areas.
Just in case test that mix.
Was the container clean when you started the mix?
Did you use the same water source?
Was the water particulary cold when you dumped the salt in?
I had one hell of a problem with what you are describing in the mix. Killed all my snails, hermits, emeralds, shrimp, and others. Still can't find my 6 line wrasse so assume that he is dead.
Test the water before use.
Thomas