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.
stinkyattic Reviewed by stinkyattic on . Wilt to Sudden death I am in DWC with plenty of air in the buckets, 4th week of flower, 1000W HPS, using GH nutes, PH stable at 5.4-5.7, nutes ranging from 600-700ppm, temp is 75-84 day, RH = 49-56%. Two days ago I notice the water level has not gone down like the other buckets which eat about a 1/4 gal per day and checked to make sure all the nutes and PH was correct. Everything checked out fine. Today all the leaves are drooping to an extreme measure as if the plant has stopped all water/nute takeup. the Rating: 5