It takes about 24 hours to fully degas the CO2, and nitrification acidifies the tank also. This is why you are seeing a 6.6. When I mix up my RO water, it will have a pH of near 9. When I do a 50% water change, the pH is pretty close to what the tank was. With 0dKH or near 0dKH, this is expected and has no negative impact on fish. My tank will have a 5.1 pH right before my water change and after, it's about 5.8 to 6.1.