I live in an area with near perfect city water. The problem we have is that fish bred in harder water have problems with osmoregulation in our water. Fish not bred in our water need to be acclimated for a longer period (I always use drip method over several hours), and often don’t survive as long as fish bred in our water.
It is my opinion that there could be several reasons including things like stress, ammonia build up during shipping (3 days is a long time), big difference in PH, big difference in KH and GH, water conditions not ideal for them, etc.
My wife and I do import lots of fish, we always ship them over night, have the shipper email us the PH,GH, and KH on the day of shipping, we measure these parameters our shelves in the water the fish arrive in, and measure the tank parameters of the tank the fish are going into to help us acclimate the fish better.
Steve