I use promix, but I keep my ph at about 6.3.

I treated it as though it were soil. My plants have all done great in promix for the 2-3 weeks.

When they're initially seedlings in rapid rooters is the only time I'll water with water ph'd below 6.0. I use 5.8 to be exact. Something I read about the slightly more acidic range being great for root development early on.