What is the nitrate additive? When you said zero nutrients, does that include phosphate?
While it may seem counter intuitive, you should feed more. Cynobacteria is adept at thriving in low nutrients. Cyno can use inert nitrogen gas in a proess called “nitrogen fiation”. I went to your build thread to understand your biofiltration but could not find those details. You have a very large tank, 4 mysis cubes is not a lot of food.
What are you doing for nutrient export?
What are you growing in the tank: nutrient recycling?