Depends on the media. pH is a tricky thing for a lot of growers, new and seasoned. In soil, pH of input solution is a damn near irrelevant if the soil is built properly. In hydro, you gotta have adequate alkalinity or keeping the solution from swinging like crazy is going to drive you insane.IMHO, It's better to not acid down water at all as in the end the medium will acid down from feed & its more of a struggle to keep it up than down.
If pH is diving in hydro, you've bad bacterial cultures blooming. It's a symptom of a deeper problem. pH should be pretty damn stable, on move on a smooth curve as EC and water levels change.
Distilled water could be used, but it's inadvisable since it's zero PPM. Even RO is pretty low but it still tends to have some EC in it, even if it's not terribly significant. You'll just have to use more alkaline buffering and distilled water just isn't economical for long term use.What about di water. Is this an option?