Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you make a 50/50 blend of what they were used to (water softener) and new (unfiltered) water, then do a 20% water change this is a very safe ratio.
I would do 2-3 changes like this over a 1-2 week period, then go to 100% unsoftened water.
If you want the math:
Do water changes where the net change in TDS is not higher than 15% more, nor lower than 10% less than what they were used to.
Example: If the current TDS is 200, then a water change with softer water could result in a TDS of 180. A water change with harder water could result in a TDS of 230.
After a few days the next water change could drop that again, 10% lower, or 15% higher.
These are conservative values. Most commonly kept fish will handle a greater change than this.
If you do not have a TDS meter, then use GH and KH to figure this out.