Flatworm
Phylum Platyhelminthes
This worm can grow to 15 mm and in most cases is harmful. The one common in aquariums and ponds is the Turbellarian which feed on microorganisms. There is over 25.000 species of this worm and many are free-living like this one on the photo, but many are also parasitic.
Fish will eat them.
Photo by Dusko Bojic.
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