HELP!! What Did I Do?!??!

[naesco]

IMO what ever happened caused the oxygen level in your tank to fall with the resultant shock and fish deaths.

With any type of material change that sees fish gasping, IME it is a good idea to get as much oxygen in the water as fast as possible.
This can be accomplished by using air diffusers, adjusting the skimmer and placing a large powerhead a couple of inches below the surface of the water and a large water change