I would suggest that you get a TDS meter (they are not too expensive). I did experiment with some pH down products but so far have found that they increase the TDS.
I am targeting a pH of 5.5 for now and a TDS of 12 or less (hopefully).
Anything below 6 pH is recommended.
If you are getting 5.0 RO water you are fine. Be very slow to add to the tank if you are at 7.4 Take a week or 2 to slowly mix. The fish will be fine with slow changes.
The whole point of low pH appears to be achieving a very low bacterial count, as these fish have not evolved with a great deal of resistance to disease.
Sorry, I would also like to add that it is also very important (low pH and TDS) for successful spawning of the paros. That is why we are here… not to just have them, but have successful spawning to prevent likely extinctions.