How are you checking the pH? Also distilled water has such a low ionic strength that often it WILL produce incorrect readings on meters that were designed for high I.S. nute solutions. Don't fix what ain't broke; your plnats look excellent.
The leaves were yellowing from hunger. It is time to start in a 1/4 strength grow fertilizer. Also, low leaves naturally yellow and drop when they are no longer needed, or if they are deeply shaded by fresh, thick growth.