I can only comment on the RO water but RO water has a GH and a KH of 0. PH can be different depending on how high quality an RO unit is and how high the original tap water is. It's not so much about using it to lower PH (although that is a benefit of it IMO) but it's about lowering your KH and GH to make harder water softer. Which in turn lowers PH. I have to use a mix of RO and tap due to certain plants I grow that just won't do well if at all in anything other than soft water. That is also a benefit to my fish which are mostly S. American. I have a dirted tank with a high peat content in the organic soil so finally almost a year down the road I am having to use less RO due to the peat soften the water.
While most aquarium fish today can acclimate to harder water/higher PH some fish like GBR's still do better in softer water. And wild caught fish, especially ones from soft water like S. American fish and others will often perish if their tank water isn't soft. They just can't assimlate well to hard water.
Another thing about RO is some city's tap water contains alot of other types of minerals that RO units remove giving a purer water. If using straight RO like some do in SW (I used to do that) you have to reconstitute the water with products to add back in the minerals your tank needs. But you can also regulate how much to add depending on what you want your KH/GH/PH to be.
