If your source water is the reason for the really high pH, I recommend putting the source water in a large container that you can add a bubbler to. Then adjust that water with acid down to the pH range you desire. This might take a couple days to actually get it adjusted and stable before you use it to do the water changes on a system that has fish in it.
I"m very familiar with GH pH Down. It is a good product and should work well for you. It is very powerful stuff so I recommend only using a little at a time.
Sorry I did find section and spent too much time taking notes before coming back here. this place is like a libary too many book and not enough time. Once again I'm sorry and thanks again. LF