Dry leaf edges point to a root problem. Usually too dry.

Beyond that, because you're growing in ProMix, I suggest you fertilize regularly rather than only "a little."

Thank you for your response Jean. I stepped up the watering a bit. I have been growing tomatoes year round for many years now. They usually do fine indoors in Pro Mix and I only give my plants a little fish/kelp and that also has worked out fine as tomatoes aren't heavy feeders and the stuff I use has most of the micro as well as the macro nutrients included. I'm thinking it's a leaf problem and I can't match it up at the 'Tomato Problem Soving' website vs. the pictures there...????