Yeah, the buffers will sometimes be good enough to make your 8.5 pH water in the 6 range or a little lower. But buffers don't last forever and can only take so much abuse. That is why it is important to use good quality of water from the get go. The buffers are really only for the changes AFTER you mix the solution and put it to use. The worse your water is, the sooner they will fail to keep pH constant. Addition of
nutrient when topping off helps, but it is best to fully change the solution on occasion. By good quality, I don't mean water that you added acid to in order to bring it down from 8.5. I mean water that measures 6-7 without additional chemicals, like rain water, RO water, or distilled. The pH being that high would be due to a lot of mineral content that could cause you other issues and don't go away just because you add acid. I hope that helps.