I am not sure what they are but I have heard that there is a rare freshwater species of a type of tube worms like the marine species but for the moving around the aquarium I have no idea how. sounds like you got a new strange creature lurking in your tank.

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The sea -- this truth must be confessed -- has no generosity. No display of manly qualities -- courage, hardihood, endurance, faithfulness -- has ever been known to touch its irresponsible consciousness of power.

Joseph Conrad (1857-1924) British novelist.