brown spots, dying leaves, purple stems

I had this a little more descriptive but my phone just deleted itall so hers it summed up. Distilled water ph about 6..liquid tester. 90w ufo with cfl's dwc. Northern lights 750ppm keeps going up so i keep dropping it more. 75 degree rez Water. I thought there was something wrong with the roots but it might be me bring paranoid. They were white but are getting tanish colored. Brown spots on top fan leaves.bottom leaves are yellowing and dying.some leaves have holes. The stems are turning deep purple. Any ideas? Oh i just changed the rez water, most of this was already there. the spots showed up the last free days. Its also about a month and a half veg. Ill put photos up soon