You will need TDS meter. I use to have 5gal tank with 3 Seiyu stones 4"x3"x4.5" in size.
TDS went to 425 in 15 days. My PFR slowly disappear. No breeding at all. Substrate was 1.5 years old ADA Amazonia.PH was 7.2
PFr's and orange neos are completely different in hardiness and living params. With orange neos, harder water is better than soft IME and TDS on the high side shouldn't bother them at all.