I agree with more light intensity and smaller photo period. 10-12 hours. Seems like trouble started with the addition of ferts.. Some ferts can cause die back if plants are not used to it or so I have heard. Depending on the type of tank top you have a cheap t-8 double bulb fixture for under $25.00 might work for you (lowes,hdepot) just not pretty w/o a diy canopy. Also if diy handy, overdriving your current fixture with an additional ballast to push more power through bulb might be an option and would take a little research. I'd stay away from fert. until plants are well established. I have not had good luck with ferts., but I'm not a plant person either. Good luck.