Root rot is a factor. Your plants looks very rough man.... Tell us your set up, your nutes along with flood times number of air stones etc.
cant just jump and assume root rot...but i guess its a good start...whats giving you this indicator? roots brown and slimy? also...whats your pH at and how old are your plants and how much nutes have you given them? to me it looks like you burned them up going to high on nutes welcome fellow VA grower
The water the plants are sitting in looks very green and murky, that's probably a clue. You should just fill the whole dang tub with hydroton, it will also allow your roots a lot more space to grow also. But it will use a lot more hydrton, and hydroton is a bitch to clean to reuse.