please help lady in distress

please can someone help started my second crop 1st one was great no probs in 3rd wk of flower lower leaves are going yellow and black at the edges ph is 5.5 using ionic for soil using directions on the box dont know if nutes or something more serious could it be when i took the clones
please help as dont want to lose them forgot to mention using 600hps for flower white florescent for veg