Hello everyone. Recently, I noticed these worms populating the filter media in my aquarium (pictures included). They are small, brown, mainly immobile worms. I thought that they were some kind of fungi or algae because they didn't seem to move at all when I was removing some of them last week. But today I observed them closely with a flashlight, and a couple of them made small movements through the filter media, thus confirming that they are indeed worms. I am preoccupied that they might affect the fish. I think these worms are eating the brown gunk that forms in the filter media and I'm not sure if it will affect the biological filter. What should I do? Is this common? Thanks in advance.