MMM. As miami said, if you're topping your tank off with a salt (sodium chloride) containing additive you will be increacing the concentration of salt as time goes by. I know Jungle Start has salt as a slime coat protectant, but there are other dechlorinating products that use other substances.
As far as that I don't know. You may try the fish store for the water tests to be sure. Maybe there are issues that their tests can measure that simple tests can't. You didn't put somthing into your dack as decor that could be killing the fish did you?