Chiefin, IMO if it was a over fert issue, we would be seeing signs of leaf discoloration....
The plants in my Ebb&Gro site system do the same exact thing when they are being over watered.. If I turn back the watering from like every 4hrs to every 5hrs (or whatever), the issue corrects itself....
Matter fact Dank, when I'm working with young mothers, that is exactly what I'm looking to see when I'm determining my watering schedules...
Ya see, in ebb & gro sites, the top of the hydroton will dry first over say 5-6" down...
With young plants their roots are not as long, so when they are not 6-8" down to get the wet hydroton, I have to flood more often.. But as they get older, they can get to that wet part, but now it's always wet because I'm flooding every 3hrs... So when I start to see that curled leaf starting, I will turn back the watering to every 4hrs and the leaves straighten back up in a few days......
Point being, that is what I see when I'm over watering...
And yes you can over water hydroton if you do not use air stones, as I don't in my Ebb&Gro res..
IMPO, it's an over watering leaf curl issue...
I personally suggest you dry them out...
You can see if you have to much salts in the soil if you measure the ppm of your water, then catch the runoff and measure it... That will tell you roughly just how much nutrients you have in the soil....