hey guys i gotta few questions about my babies, im also gonna post this in the infirmary section...but one of my plants fan leaves tips are starting to turn yellow, anyone know wat the problem could be? theres some pics below of it

also, this is my first time growing and everything so i barely know anything about ferts, nutes, etc. my plants started off on 18:6 with 6 cfl's and they were germinated and planted a month and a half ago...im thinking about switching to 12/12 in two weeks for flowering (because i have no where to put them in a few months) but i was wondering if anyone in here could give me a schedule of nutes or ferts or w/e...and a preferance on wat brand and everything...any input would be great since im such a noob! lol...thanks!:smokin: