You are welcome,
As Jonathan noted rtn and stn are not well understood. A variety of factors may contribute, including but not necessarily limited to: alkalinity, ph, hypoxic conditions, flow, sg,bacterial activity, temperature,organic carbon buildup, a lack of consumable food,alelopathic ot other toxins, high nitrate and/ or high phosphate.
High nitrate and /or high inorganic phosphate(PO4) do ,however, cause difficulties in excessive zooxanthelae production perhaps increasing oxygen in the coral to unhealthy llevels and in the case of PO4 inhibit calcification. These stressors could trigger stn or rtn particularly in combination with another underlying problem but I don't know have anyway of knowing that. FWIW,in my past experience when nitrates climbed over 30ppm and/or PO4 over 0.3ppm stony corals darkened and closed their polyps and sps eventually exhibited stn or rtn.