Geezer From The Freezer wrote in message ...
Brian,

I've been hit with Planaria in my tank recently. They can be as
little as a couple of millimeters and as long as a centimeter
or more. They typically look like white hair floating in the tank
but when they swim, the wiggle.

Almost certainly not planaria (triclad platyhelminths); most likely
either free-living nematodes (roundworms) or oligochaetes (relatives
of the earthworm and Tubifex).

The typical planaria are flat, broad, somewhat slug-like creatures,
totally without segmentation, normally ranging from 0.5 - 3 cm,
although there are species reaching 60 cm. Color varies wildly, but
usually brownish or whitish. Typically they look like something like
this: http://www2u.biglobe.ne.jp/~gen-yu/pl7s.jpg
These move with a gliding motion; may "swim" if dislodged from the
substrate. And, as I said, they're flattened, hence the name flatworm.
They are scavengers and small predators. They do appear in aquaria,
but IME fairly rarely.

The best known nematodes (roundworms) are parasitic, but most species
are free-living, feeding off plants or bacteria. Nematodes range from
0.1 mm to 8 meters in length, but nearly all of them look like this:
http://reflex.at/~biolab/Fotos/Evert...x/nematode.jpg
Move with a snakelike, thrashing, motion. Probably the most common
animals on Earth; nematodes of some species or other are basically
guaranteed to be present in any aquarium. Familiar nematodes are e.g.
microworms and vinegar eels.

Oligochaete annelids, to which earthworms and Tubifex belong, look a
bit like nematodes, but can be separated from them by virtue of being
segmented (difficult to see on very small species, of course). They
range in size from 0.2mm or so to 2.5 meters. Typical example:
http://fuv.hivolda.no/prosjekt/joste...d/250fabor.jpg
These are detritus eaters. Move like, well, earthworms. IMO
oligochaete annelids are the most common worms in aquaria.