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Detritus worms are a common problem in some tanks that do not have clean enough gravel; they are a natural part of the aquarium ecosystem however if large amounts of organic mulm (decaying fish food + fish waste) is left in the gravel this can cause a population explosion. The worms are hair-like in appearance and usually vary from a couple of mm in length up to a couple of cm. They can be found both in the water column and on the side of the aquarium; if free-swimming they usually squiggle around or swim in graceful "S" movements.

Planaria is a species of flatworm that is harmless to fish. They usually hitch hike in on plants and are often pale grey, white or black. They are thicker than detritus worms and can be much larger. They often have an arrow shaped head and aren't usually seen floating around the water column, they can sometimes swim on the underside of the water surface (pretty cool!).

Cylops and other microfauna are also harmless to fish and are often found in aquariums that have new plants (they are hitch hikers, too). Seed shrimp, clam shrimp and daphnia can come in via fish food. Some fish will eat these little beasties but if they worry you you can try doing a series of water changes, this will remove any in the water column.
Some microfauna species are absolutely tiny (like you described) and look like balls swimming around the water erratically, others dart through the water in random spurts. Some of them have forked tails, some can be quite large but in general they are harmless.

You can always try googling images of seed shrimp, daphnia, clam shrimp, detritus worms, hydra, cyclops and planaria. This may help you identify the small creatures through process of elimination. I've accidentally and purposefully raised a variety of these critters and while worrisome at first you may eventually find yourself happy to have some of them in your aquarium - I love watching Kaze hunt cyclops, there's something awesome about seeing such a natural behaviour.
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