You need an accurate low range Nitrate / Phosphate test kit(s). Coral does need some Phosphates & Nitrates. As those values decrease they become sensitive to higher Alk (8.5+) and light. Good idea on turning the light intensity down.
You can research ULNS (Ultra Low Nutrient System) to get some good info on what is happening. Every system is different so it is hard to say one particular issue is causing the problem.
Red Sea, Salifert, and Hanna (Po4) have good low range test kits.
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90g Mixed Reef
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