You're growing in soil right? It looks like too much nutes to me also, do you have a PPM meter and if so what is the PPM of your nutrient solution? I have some G13 plants growing in another hydro set-up and after I raised the PPM of my solution, they started to look just like yours with burnt tips and curled up leaves, after I lowered the PPM a little the leaves perked up and the new growth has no burns on the tips. Also, how often are you watering?
Last question, what size pots do you have them in?