If you can find out what is raising the pH to 8.0 and stop that, the normal biological processes will tend to lower the tank pH. Starting at 7.0 you ought to get to 6.5 pretty easily.
Basically, in trying to lower pH or soften water, what you need to do is to remove things from the water, not add.
You can use filtration to remove those things, like an RO unit ($$$). Or you could cut the water with a bit of distilled water, but not much, maybe up to 25% or so.
If you can get pure rainwater, then filter that to remove pollutants (floss and carbon) you can add some of that to the tank to dillute the salts and minerals and such that make the KH and pH what it is.
Peat works because it absorbs the minerals and it adds acids. Search on Peat and you will fins info on how to brew up peat tea to add to the tank to lower the pH and KH.
But, all that is worthless if there is something in the tank that is leaching carbonates into the water. As the pH falls, the leaching will be worse.
I think you need to get some muratic acid and test the gravel in the tank, the vinegar test is not dependable.
Hmm, does someone know which test kit it is that has acid in it? I vaguely recall that there is one kit, like KH or GH, maybe?