So if I read correctly, you set the pH at 6.5 and it dropped to 6.0?
Is there a funky smell associated with the water?
Is it being aerated in any manner?

When I have fert issues my first plan of attack is to strip down to my BASIC fert, no supplements at all, and add supplements back one by one. It seems as if a pair or more of your ferts/supps are interacting but I don't know which. Rhizome is the man on this one.

You are not watering with lights off at all?