your nutes are freezing ?????? how are you keeping the plants warm when the lights are off?
just like it is easy to over water when they are little it is easy to over fertilize. they don't need food with every watering when they are small and/or the nutes DON'T need to be full strength.
coco nutes and soil nutes are not exactly the same. i've read that coco is high in K on it's own so the Ca and Mag levels are higher in the coco formulas to balance it out. i don't think that means soil nutes won't work ......... you just have to be aware that they are not "dialed" for coco.
i would not go running to the grow store yet! ........... read some stickies and ask some of the coco headies some questions. the good thing about IC is that nobody is trying to sell you anything
good luck!
My grow box isn't very big so I just put an iron next to the intake and the temps are 23°C, but outside the box where I keep all my supplies and shit. that's the problem with my country, we don't have hydro stores, I got all the stuff I have from a supermarkets and similar places or found at home, there are some gardening stores but they're very poor in choice, gonna check one more store tomorrow and see maybe they have something usefull. right now I have some nutrients with 5-2.5-2.5, and schultz 10-60-10 my nutrient sollution is half of each and I mix them with half water when watering. I am also thinking of using organic teas as fertilizer, but prolly thats for the future grows.
I moved here from rollitup, and loved it here, I get good friendly advice, and no negativity for now