We swim regularly in natural water, mostly in the Maasplassen. Especially at the moment with this extreme heat, it is important to pay attention. So a very good blog and a nice tip from the app I didn't know it yet.!
Indeed, I have seen such a paint-like green or blue layer on the surface often enough in the summer on the water of the Zuiderpark in Rotterdam. I don't like swimming in open water anyway.
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