Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the tap water is OK, just hard water (high GH and KH) then try blending RO + Tap. Then test to see if the GH suits the fish.
If the KH is anywhere near the GH level, then the pH will also highly likely be in the right range for the fish.
If you are keeping black water fish, then I would filter the blended tap + RO through peat moss to add the organic acids these fish like. This will probably drop the pH a bit more.
If you are keeping rather adaptable fish, and it is the plants you are worried about then research the plants' needs. There are a few specialty plants that really do require low mineral levels, and the resulting low pH. Aim to keep these plants happy, and the other, more common plants will usually adapt.