Nematomorpha: Gordian worms.

Rarely you might see extremely thin wiry worms up to many inches long, called "horsehair worms" --or "Gordian worms" because they can tangle in knots. They would likely be secretively wound among the plants, including floating plants. These representatives of a small and obscure phylum, Nemata or Nematomorpha ("threads" or "thread-forms"), would have come into your aquarium among pond-raised plants, for they are parasites of insects, such as grasshoppers or crickets, or water-loving flies like mayflies. You might rarely find them as parasites in bloodworms (Chironomid midge larvae). They do not trouble fish.

These Gordiids don't feed as adults, but sustain themselves on stored nutrients, for up to a month sometimes. The fertilized females lay minute eggs in long gelatinous strings that get wound round plants. The eggs will hatch into microscopic larvae that drift to the bottom.

Gordian worms aren't well-documented, so they're a bit mysterious. The most detailed sketch of gordian worms is at the homepage of Ben Hanelt, PhD student at University of Nebraska, who's studying them.

Dr Mark Wetzel and Dreux Watermolen's description of the lifestyles of Gordian worms is also trustworthy. They offer a long list of potential hosts for the immature worms:

"As larvae, horsehair worms are parasites of insects and other aquatic and terrestrial invertebrates, most notably grasshoppers, crickets, locusts, katydids, and beetles. Other hosts include caddisflies, dragonflies, spiders, millipedes, centipedes, crustaceans, and leeches. Host specificity has not yet been well documented. Other hosts include representatives of vertebrate groups."