You know what's wrong, you said it. Stabilize your nutrients. I do not use hydroton so I do not know first hand but I think you need to stabalize it. Did you soak it before your grow? I heard it is a bit hard to keep low. You did not say what you are feeding them exactly. I am assuming you are following the bottle. You might want to mellow it out a bit. You should not even be at 600. I would say 200 or 300 (ecx500) at the stage you are at.
I notice when I feed my plants too little the TDS goes down after a feeding. When I feed them too much the TDS goes up. I like my nutrients to be at a place where there is a very slight drop in TDS. Start small and build your way up till you notice that drop. Check before and after a watering. I would also flush your medium with pH corrected water. Around 5.0 (help stabalize it). Worked for me before. After the flush start small with the nutes and aim for a small drop. Not a big one. No drop is ok too. When it goes up. that sucks.
good luck.
post up how they do. You should notice a change soon.
It is good you at least noticed what was wrong. Your grow looks really nice. Genetics plays a huge roll in the grow, keep that in mind since you put a lot into it. Bag seed might not be the best idea.