Have you started using hot tap water to temperature match for water changes? Hot tap often has higher copper concentrations, if you have copper pipes anywhere in the system.
ill put a 5 gallon bucket in my bathtub, and put on the hot water, and fluctuate the temp from hot(not steaming hot..just like i guess warm) to cold to make it as close to the tank temp as i can..
because, the other fish store got mass amounts of fish for the big huge sale, and they were probably already stressed or sick, so it was a bad stock im guessing. if the new fish i just got, do fine in my water, which 6 hours later from my last post they are still fine, it will help prove, atleast in my head that its the stock not the water, cause the other barbs i hve would be sick right now.