Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you have a TDS meter, then make that match.
If not, then all you have to go by is GH and KH.
Make those match.
Get close, then drip acclimate.
While the fish are in quarantine do enough water changes to drop the GH and KH to match your main tank, but let that happen over about a month. The fish can handle it that slowly. They cannot handle that big a drop in just an hour or two.