Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Actually, it is the other way around.
Fish get used to the water they are in. The TDS, or total dissolved solids. In this case the minerals and salts.
The cell wall is like a semipermeable membrane. When fish are in low TDS water the water in their cells holds more salts etc, and more water keeps trying to enter. The fish metabolism gets used to the hard work of pumping out that extra water. When this fish is placed in slightly harder water they do not have to work quite so hard. This is easier on them than going into softer water.
When fish are kept in high TDS water less water keeps trying to enter their cells, so their metabolism is only geared to removing that much water. When they are placed in water with less minerals and salts than they are used to more water enters their cells, and they are not used to having to get rid of this much. This is harder for them to handle.
Salt water (Marine) fish are different again. Their system is geared to holding all the water and getting rid of the excess minerals and salts.
To answer the original questions:
Fish need the minerals that we measure as GH. Calcium and Magnesium.
Softened water usually has very low Ca and Mg, but excess Na.
Measure the TDS of the tank water before a water change, and after.
If the TDS is not more than 10% lower, or 15% higher, then even fairly delicate fish are OK with this sort of water change, as long as there is still some Ca and Mg present.
If the water change creates conditions where the TDS changes more, or there is almost no Ca or Mg, then the fish would be in trouble.