That's a big plant [read:big root mass] for standard-sized DWC. Root rot is most likely your culprit; you can do a couple things to try and save your plant, and the rest of the crop:

-Raise the lights, or better, switch to flouros right away to reduce water loss, and personally I'd turn off the CO2 tank. You want to slow them down a little because fast metabolism means fast water loss, which you can't afford with nonfunctional roots.

-Next, see if you can't manually insert some more air stones INTO the center of the root ball- the areas with least access to oxygen harbor the most potential for rot to form in those nasty anaerobic conditions.

-Run a 'zyme product to remove all dead material from the reservoir, following instructions PERFECTLY and changing the res a couple times because it will release a lot of useable nutes into solution- which you actually DON'T want, since it's at an unknown level/ratio.

-Explore your options for a larger reservoir. High biomass and low volume mean that chemistry is VERY difficult to control and you will have all manner of troubles.

Weedhound, can you link to where you went through your root problem before you switched to livestock feed tub DWC? I think there's enough material for a DWC root rot sticky in there, because the process and solution are really well documented!
(I'm sending good vibes your way too! )

EDIT: WH is right about pH and CalMAg too.