Could be many things. beginning of Nute burn ( try backin offf the PPM's a wee bit), ph fluctuation (your ph seems fine), ozone damage, potassium deficiency, calcium deficiency, fungal pathogens, viral pathogens, fugus gnats etc...
fungal pathogens often causes stem rots, & round leaf spots, concentric rings, necrotic discoloration, or wilt.
viral pathogens can inhibit chlorophyll formation, causing degrees of yellowing or redish/brown mottling, stunting, distortion, or dieback of part of the plant. Viruses usually debilitate rather than kill, as they are parasitic. or
Nematodes are microscopic roundworms that cause disease-like symptoms. Stem nematodes feed on stems and cause shortening of internodes. Root nematodes feed underground, damaging the root system ; this leads to moisture and nutrient stress which shows up as wilting and stunting or redish/brown mottling. Foliar nematodes cause angular leaf spots. or sucking insect feeds on plant fluids & can cause leaf spotting or stippling (aphids, leafhoppers, spider mites, other bugs), fungus gnats in a DWC system has been know to cause a rust looking affliction to the leaves from larve feeding on the tinyest of roots
inspect the res closely for almost microscopic critters feeding on the roots, or dead in the bottom of the res
viral pathogen or fungal pathogens can usually be dispatched with the use of 70% strength Neem Oil.....this I would try first, as there should be no ill affects to the plant with this treatment
I hope this helps you isolate the problem, keep us posted, with what you find as you begin to isolate the problem