I would say yes. Although a water change shortly after the addition wouldn't hurt. That is a good point though. You wouldn't want to add the water from the lfs, so why add the water from LS. I suppose you could put a hole in the top of the bag to pour the water out of slowly, but I would think that the bacteria would be at risk. Better to do a water change like I said to be on the safe side. HTH--Bob