Hello Peter,
Krause (1998) describes that breaking carbonate hardness with (strong) acids changes the Ion-composition of the water. As a result some fish-species do not breed when the Ion relativity is too far away from standard water. He uses the german term “aquaristischer Kuntsfehler”. You say that Parosphromenus are not sensitiv in thar way to “Eichenextrakt”, do you have experiences with HCL? I used “Eichenextrakt” in an experiment without fish and it led to an strong increase of the conductivity, so I dont use it to lower pH. I only use peat, conductivity rises only a little bit. But it takes a lot of time untill pH goes down. It seems that fishes help to lower pH (more nitrification?, CO2?).
Greetings
Patrick
Krause (1998): Handbuch Aquarienwasser 6.Auflage. Bede-Verlag