Correct. Say your target TDS is 100 ppm and your tap TDS is 400 ppm. You'd mix 75% RO with 25% tap. With domestics you may not need to adjust the water at all. If you feel you need to drop the TDS I think 50/50 RO and tap will do fine.So I assume for the TDS i would in a holding bin mix the RO and Tap and then let it sit heated and with an air stone in it a few days prior to the water change but what would i do about the PH?