My DD's had very similar clawing, Glasgow, and with a very, Very light N regime. With a great yield, I might add.
I think Pete is on to something with the water. Especially with the tier you're using, figuring out the water is key, as is making sure those tier pots are holding adequate H2O.
My gals also wilted quite a bit after lights out, but perked up once the timer turned on. Do they seem to recover during the day?