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I've had little worms appear in one of my tanks before. Sometimes they just float around in the water. When I got some Neon Tetras, the Neon's would eat them. I'm sure they will disappear after a while. I have no idea where they come from though.
Nematodes and free swimming planaria are caused by an overabundance of nutrients in your tank. Usually from overfeeding or underfilter/vaccuming. Make sure you keep up on regular maintenence, and you should see them go away. They can become quite unsightly, but are harmless.
I would never recommend livestock to fix a problem.. its rarely wise to stock this way, incopatablity and increased bio-load are just a couple of the problems adding more fish/inverts can have.. be it algae or any other aliment the tank may have, proper maintenence is the key to a heathy tank..