nute up-take will raise the pH slightly higher,1.5 is a little extreme,it might have to do with the rockwool not being stable enough,if it's not stabilized in a pH water solution of 4.5 for at least 24HR's,it will still release alkalines in the reservoir afterwards,next time you stabilize it just let it drainout on a cappilary mat,do not rinse it ,if you do rinse it out, it will defeat the purpose....bonbon