I have had an awful time dialing in my new LEDs coming from T5s and my Zoas were doing the same (growing and colonizing fine then just randomly shriveling, closing and melting)... I tried everything you are trying except the Vitamin C. I have a 125g tank that is 24" deep. my Radions are about 13" off the water with TIR lens and all parameters were normal. I was convinced that the LEDs were not very strong (i guess because they are not hot) and had the Intensity set at 70%. I guess I was burning almost everything up especially Zoas. After months of tweaking, I am at 35%, mostly blue and very little white spectrum and everything seems much happier. Montiporas, Shrooms, Frogspawns/Hammer and Acros are growing and Zoas at the bottom look much better. I think my next tweak will be to slowly raise the lights for better light spread but IMO and having dealt with my own frustrating Zoa problem... I think its a very fine line with LEDs Intensity and Spectrum!! And Zoas are supposed to be "Easy".