I could be wrong, but I really don't think water could be "too hard" for crayfish. It was probably not the best idea to add salt to the water, but you can rectify it by just doing some water changes and using whatever methods you had been in order to get the hardness back up.
As far as the lack of appetite, they often won't eat when they're getting ready to molt. For females, they also typically don't eat much when they are about to lay eggs or are berried. If you notice them not eating, just hold off a couple days from feeding, see if they molt, and try and feed again a few days later.