Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish want low GH, the pH is not the issue. Cardinal Tetras, for example can get a build up of calcium in their system and die. GH measures Ca (and Mg), pH does not.
Repeat your experiment with RO, but measure the GH and ignore the pH. If the GH out of the tap is so high you might end up with around 75% RO and 25% tap water to drop the GH enough for these fish.
Then add peat moss to the filter.
Once you find the right recipe introduce the change slowly. The goal is to change the GH by not more than 2 degrees at a time, less is probably safer. You could make such a change twice a week. Give the fishes' metabolisms a chance to adapt to the reduced TDS.