Your ppm's are to high, I have hard water about 250ppm and mine show similar issues in the seedling stage but not as bad. Your ppms should be around 200 or lower before the cotyledons start to fade. Once the leaves show signs of burn of discoloration they will not turn green again unless the cells are still living. Unfortunately u picked a very advanced growing method, and it takes advanced knowledge for it to work properly. It's a trial and error process with fog along with proven methods of use. You cant just guess the ppms and have it run every so often and expect good results.