Ok I have personal issues with hydroton myself.....I think it leads to fert build up issues. I used it for about 6 months and finally changed but lots of people seem to have no problem with it so sometimes I wonder if it's just me.

I have pretty large plants in waterfarms (about 2 gals apiece) and my ph doesn't start rising like that until about week 4 of flowering and on and I used to have to adjust it constantly. Now I recirculate the nutes and that helps keep the ph down but it is considered normal pretty much at that point.

However....you aren't at that point and so yours should not be rising like that unless they are suddenly taking a huge grow jump or something. So it's pretty much up to you to watch and see if the nute level is too much which can also definitely cause ph to rise. If you want to find out for sure....drop back your ppms and see if your ph stabilizes. If so....stay on the lower side. If not....I am still wondering about the metal in your system and did you use a flushing agent when flushing the system?
Those would be my other two thoughts.