I dont think you can do anything about not washing the hydroton at this point. Even if you didnt set the PH on the hydroton before you started using it.....eventually it will stablize with use.
I think you have something else going on here that is causing the ph to rise so rapidly.
The ph for you water right out of the tap is on the high side.
I would cut your nutes in half till you figure out what the problem is.
I use Vita Grow nutes...........and have used them for years and have never had PH problems. Most people in the forums dont even seem to be aware of Vita Grow. It is very cost effective and yealds very good.
Seems to me most people who have PH problems are using Foxfarm products.
I have grown plants perfectly happy at a ph of 7. Though I usually will run my system at the high 5's.
After you havest this crop clean your hydroton and get all the residue out of your buckets.
Good luck.