I have not really found that anything worked satisfactory regarding lowering ph and keeping it low, untill I learned of the product called ‘eichen-extract’. That is really working.
Before that I found that even with reverse osmosis I had to fill in lots and lots of peat or peat granules in order to lower the ph measurable, and it took long time.
I also found that having any kind of gravel is working against low ph, – many things that you have in tanks, such as gravel, will contribute to making the ph rise again. For this reason I dont use that anymore, but rather spagnum / peat, or just lots of leaves, or spagnum granula for bottom. This seems to work well also.
I dont actually think its your RO filter operation which isnt working, – it sounds too familiar 🙂
Lowering ph can be really tricky and not that easy untill you find just that thing that works for you, – and if its leaves and peat, – you really need to use quite a lot for it to work.
And I guess small things can influence it so that ph slowly rises again, – like the food you put in the tank daily. For this reason I always rinse the artemia, first in normal water, then I use RO water to flush them out of the tiny net and into a small container. Then I use a syringe – but I dont want to everyday put like 5 ml of hard water into the small 12 liter tanks, – that I think would eventually bring ph up again. It takes very very little amounts of hard water to change the water
parameters.
And I would say, – that, yes, – maybe paros can live in water of ph 7, but they thrive much better in lower, – for a long time I kept it at 6, couldnt get it lower, – and a few fish did produce fry, – but now, when I am getting used to 4.5 its so obvious that they thrive much better, – a side ‘problem’ is that you do get too many offsprings. 🙂