Yes, Bernd, you are right! By removing CO2 in photosynthesis the plants can raise the pH. Thank your for the reminder, I think that solves the puzzle of why my pH is not going further down.
The questions are:
Is pH 5.5-5.7 low enough to minimize bacteria load that could harm the eggs/larvae? If not, should I remove plants to lower pH? or use acid to lower pH?. I would like to keep plants if possible – I see many of the good paro breeders have quite a few plants in their tanks – are they getting low enough pH, even with the plants? One thought – at 55µS, maybe my pocket pH meter gives an incorrect reading (to high) due to low conductivity? I will re-calibrate it soon and see if the reading changes.
And, is conductivity of 55µS=low calcium and other minerals low enough for eggs to stick to the roof of the cave and develop? I think I can lower it with more RO water changes and will do it.
In any case, for the “keeping phase” perhaps the current levels are OK? For breeding they may need to be lower. One step at a time, the pair must grow to a compatible pair, we’ll see how it goes.