GH does not directly influence pH, but the calcium and magnesium ions responsible for the GH titer in natural waters are commonly associated with carbonates and bicarbonates (KH) which do affect the pH.
I just read an article on GH and the only "reasonable"(besides RO or RO/DI$$ ) was with peat moss. I'm not sure I want to try that yet. The reason I wanted to lower it was most things I read said SA cichlids and tetras preferred softer water. I don't know I have been using same water for 10 mo. now so I guess I leave it the way it is.
Thanks for the response
If you are breeding blackwater fish, then you may well need to reduce TDS including Ca++ and Mg++. If you are maintaining fish in a mixed tank, adapt the fish, not the water. It is easier, cheaper, and a lot safer.
One of the reasons that books will tell you that fish need a certain pH is for breeding. Some species eggs just cannot physically hatch if the pH is above a certain point- it hardens the egg case or some such.
But for fish in community or even just decorative tanks egg hatching is not an issue- eggs would have been consumed by an opportunistic gobbler long before hatching anyway.
What I'm trying to explain (before coffee!) is that your fish can adapt easily to a change in pH- it's just that they won't be hatching any fry.
And once you begin mixing RO water with tap to lower the hardness- you will have to match that mix every time you do a water change. That gets real old real fast.
That in combination with having to worry about soft water containing less buffer and keeping you that much closer to a pH crash makes tap water look better and better.