We used distilled water and Brightwell substrate and our GH & KH fell to 0, Ph went to well below a 6.0 (lowest our kit went) and they looked awesome! I think they would have easily bred, but the tank was meant for CBS and they did not enjoy the parameters. Thus the experiment ended. I'd recommend using distilled water for small water changes and it will start falling. Remember that bacteria no longer function below a 6.0, so it will stay as Ammonia, but Ammonia in a lower Ph will shift into far less deadly Ammonium which plants can uptake or water changes should eliminate. Nitrates and Nitrites should never register. Chili rasbora are found as low as 4.0-4.5 so no real worries for going too low in that regard, the lower and softer you can get the better and the more colorful they will get. Ours were bright red and far more active.
"In the land of the blind, the one-eyed man is king" Desiderius Erasmus