Tomato leaves 'coiled up'?

[widespreadpanic]

I have several plants in my hydroponic ebb and flow system that have older leaves curling slightly upwards, and new growth rolling or spiraling together.

New growth spiraling/coiled up.



Leaves curling upwards, but no sign of yellowing or spots:



Plant seems stocky:



I am obviously worried that this is a disease/virus. I'm growing heirloom tomatoes such as Cherokee purple, Green Zebra, Brandywine, Black Krim, although pictured is a Better Boy hybrid.

Could it be that my watering schedule of every 2 hours during the day is too much?

Details:
My E.C. is 2.0, my P.h. stays around a 6, my nutrient solution is kept refrigerated at a constant 67 deg