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Black Spots in Tubing

In the Kitec tubing thread, there is mention of black spots.

Have these been seen in Wirsbo tubing? I have 5/8" Al-Pex installed, in just outside the supply manifolds, I can see a little bit of this black spots. They seemed to come sometime after initial system was filled and turned on...but don't seem to be changing.

The system is closed, mixing injection pump, temperature max for the manifold is 115. So that's not a problem. System pressure is 10-13 psi. There is some anti-freeze...

Any links to dicussion about this someplace? Thanks.
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