check pH, check tds.. rule out easy stuff first.
consider recent behaviour -- if heavy on nutes, you can safely irrigate for a bit and see what happens.
Try light at 24"... in vege with 18hours you can give less light to provide same energy.. check out my DLI chart in week 3 of a diary -- either.. if not week three try week 2, lol. it's there somewhere in each one. find umol/s of your light and look up what DLI you provide.. if in the deep red, dial it back. either way, you can likely rule out photons as the cause.. HEat is another issue to consider from your light, though.
some canoeing, which is typically heat related.