Curling tomato leaves

[platinumethics]

Hello everyone

I'm seeing some of the leaves of my tomato plants curl in early stages. The plants themselves are bright green which cancels out some description requirements for some issues but I'm not experienced in tomato plants so I don't want to come to my own conclusion.

As far as the setup its a simple DWC with air stones in the reservoir. I'm running a nutrient mix at about 1tsp of GH Grow per 3 gallons of water, the reservoir is an 18 gallon. The water was pre-filtered when added to the reservoir. The temperature never gets below 65F and never above 78F. I've kept the water level right where I need it and the root growth has been phenomenal, really thick roots and white as can be. I've checked the PH everyday and have it balanced between 5.5 and 6.0. The lighting is 18/6.
I'm growing an heirloom variety.

I'm including some pictures.
Any help would be appreciated!