Ok I am pretty sure that’s caused by over dose of iron! In my tank after I suspected iron I took some water to a lab and got the reading that said to much iron (I don’t remember the values but it was too high).
I recommend using dosing pumps…. In my tank I moved to dosing pumps for everything from vsv to ca and alk and all the elements such as iron, potassium ets….
That’s the best you can do! Always on time, always the right amount …..
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