@Ace Spoddity
I was thinking of nitric acid in the 80%+ range, but at that % it is fuming so be damn careful with it.
You may be right that there is a combination of things in GH and in fish tank PH down.
There is more than one way to skin a cat, and I am just giving my opinion that Sulfuric acid when used CORRECTLY is a very stable and cheap PH down compared to commercial and grocery store products. Baking soda can be used as PH up in a pinch but should not be used long term if one needs to constantly up the PH due to it having sodium which is a part of salt.
I have also used the fish tank liquid chlorine remover as it works almost instantly to condition tap water...but if one has the time to let the water sit and bubble over night, that is one less chemical to have to buy.
Good info though.