[Simons]

I share others comments, loosing your fish is never a pleasant experience. Sorry for your loss, and it really is a loss.

As for the reason, I also echo others comments, I highly doubt it was 'the salt' specifically...I also, purchased IO salt during the boxing sale and have done many water changes with no such casualties.

I think, it is just easy to blame the salt because, you just performed a water change and then the result was the death of your livestock.

I would think it might well 'could' have been the water change which would mean it may be your salt water mixing container contaminant...but that doesn't rule out other possibilities, was there a temperature issue overnight? did the pump(s) stop due to power failure and the water became anoxic? Are you dosing and did something happen with the quantity or quality of the agents?

There are lots of things to look at...I will agree though in the end it may very well be the salt but I doubt it would be only your specific bucket...