[Peter Finke]

Patrick, I know Krause well, his books and him personally since the seventies. His views are somewhat personal; I followed some hot discissions he had with Kaspar Horst. Krauses views are fully attached to the problems of the normal planted aquarium with average water. Blackwater or peat water aquaria are to be taken quite differently. What in the normal tank is a “Kunstfehler”, could be quite right in the aquarium with fish which need low pH. (By the way: Parosphromenus don’t “need” low pH, but there is hardly another means of keeping germ growth beyound a tolerable limit, in nature as well. The licorice have adapted to those conditions). You could breed P. ornaticauda for instance (which is a more sophisticated species) with pH 3.0, but with pH 6.3 as well, the latter only if you succeed to keep the germs at a very low level (which is difficult with a pH like this!).
Surely, there are different qualities of “Eichenextrakt”, too, as there are different qualities of traded peat. I admit that. Indeed there are some which let the conductivity rise, you are right. But mostly there are no problems like this. I use that brand which is called “following the receipt of Hans Stein”. And you could use pure phosphoric acid. But once more: In a planted aquarium with many active plants things are to be seen quite differently. I should never work with acids there.
I don’t know whether fish “help” to lower the pH, and I have no experience with HCl.