the interesting thing about salinity and fish is that fish can handle quite easly rapid drops in salinity. Physiologically, their bodies (i.e. kidneys) do not have to work so hard at conserving water. Often ehan fish are ill, dropping the salinity can help in their recovery. For instance with a parasitic infection such as ich, dropping the salinity helps to reduce dehydration often associed with this disease (e.g. sudden wasting and thinness may be attributed to loss of moisture through interuption to the fish slime coat as a result of the parasite.
However, raising the salinity is a different matter. Raising the salinity has to be done over an exteneded time as the fish has to "pysologically" adapt the the increases in salt by altering its biochemical processes. Increasing the salinity causes stress in fish when done suddenly. Alternatively the dropping the salinity by the same margin causes little if any stress (within certian limits)