is rockwool your actual grow medium ?
i've read it's really unstable as far as ph . i just use it in starter cube form and put it in hydroton once nicly rooted . my ph only jumps once usually like 12-24 hrs. after a full nute change . i tend to start out at 5.2 now because i'm a lazy high-on and i know it usually jumps a full point =1.0 in that amount of time so it ends up at like5.9 - 6.2 or so .
peace
crispi