Hi, in my eyes low phosphate is responsible for the slow growth. According to the latest findings of a doctoral thesis from 2017 at the University of Florida nitrate can damage your corals with low phosphate concentrations. Adding nitrate with low phosphate concentrations may result in bleaching and burning of coral tips, montipora plates and in general of parts exposed to light. All scientific findings under controled conditions (aquaria) of the last few years and my own experience say the same. I recommend to feed the fish more or to feed the corals.