White snowflakes

[wisefinger]

Hello,

I have an aquarium of 400 liter that has been running now for 6 months. I have an external professionel 4+ 350. I have 10 jung stendker discus about 10-12 cm in size. I'm currently changing 1/3 of the wather 2 times a week.
The fish are doing great but the water has become clouded. It seems there are small white flakes/spores/fabric floating in the water. No mater what I do, I can't get the water clear. Does anybody know what these white flakes are. see picture. Any tips/suggestions would be much appreciated.

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