water change causes cloudy water and residue (fish distress)

I have had a problem recently during water changes. I have kept a 55
Gal freshwater tank with silver dollars and angels. Water changes are
made with a python gravel cleaner, and I refill right from the tap, then
add chlorine neutralizer and some freshwater aquarium salt. For years I
have had no problem, but the last few changes have resulted in the
aquarium clouding up within several hours of the change. The fish are
also distressed and stay at the surface with rapid gill movement. The
next day the matter is corrected, fish back to normal, and the white
substance that had clouded the water is now a white residue that settles
out on to everything - including the glass, which I scrape off with
the algee magnet. The water is crystal clear at this point.

My water is hard, and I have used a water softener. The filler hose
from my kitchen would take part un-softened water (from the cold water
tap) mixed with some softened water from the hot tap that I adjust to
approximate the temperature of the aquarium.

For years I have not has a problem and the fish have always been better
after water change rather than worse. I don't recall changing anything
that I've been doing for years.

The chlorine neutralizer is stuff I mix myself with sodium thiosulfate.
But again, I've been repeating this process for years and have had
no problem until recently. This appears to be more of a chemical
reaction of some type than a bacterial. Any suggestions would be
appreciated.

Thanks,

jr