It could be a combination of things, higher nitrate and Kalkwasser. Zero phosphate is not good. I lost half of my corals a few months ago when I let my phosphate get to 0. I was dosing KNO3 at the time and as the KNO3 increased the corals, the algae, the coralline algae all starting using up the phosphate and it crashed. I was running GFO at the time which made it worse.
I need to add nitrate to colour up my corals. So I am now adding KPO4 to keep my phosphate at a readable level and adding KNO3 to keep my corals colourful.
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