Harmless free-living nematodes in the aquarium

Nematodes. Often aquarists are startled the first time they notice minute threadlike whitish worms thrashing from side to side (front to back, to be strictly accurate) as they make their way through the water. Once you've seen this characteristic movement, you'll always recognize a nematode. Nematodes have no ring-like muscles to constrict or swell, like a segmental annelid worm— think of an earthworm moving in the annelid way of coordinated waves of constrictions. Instead, nematodes just have clusters of longitudinal muscles, which give them these distinctive waggling moves.
 
Nematodes are roundworms, not flatworms. Though their size ranges overlap, you'll never confuse the two, because their moves are completely different. You'll never see a nematode gliding along the glass or under a still water surface like a planarian. You'll never see a planarian thrashing in the water like a nematode. One critter you might mistake for a nematode, though, is a naidid worm.
 
So far, biologists have described almost as many species of nematodes as they have of fishes, and no one cares to speculate what percentage of the world's nematodes haven't yet come to scientific notice: are there 200,000 species of nematodes? Fertile soil swarms with nematodes, which live in the film of water covering each minute grain, and so does the freshwater biofilm, though by far the majority of them are microscopic.
 
Any nematode you see in the aquarium is not a parasite of fishes at any stage of its life-cycle. That's reassuring news because most of the nematode press is pretty negative: they attack humans (river blindness) and our pets (dog heartworms), they are parasites of insects ("good" nematodes!) and attack plants ("bad" nematodes!). Nematodes get a bad rap because so many of the parasitic ones cause human diseases and the economic stress that is often a motivation for scientific funding. The nematode Camallanus is an insidious intestinal parasite of fishes; hit the highlighted link to find it and the less destructive parasitic nematode Capillaria. (You never see these loose in the aquarium.) But a nematode, Cænorhabditis elegans, was the first multicellular animal to have its complete genome deciphered, leading to a Nobel Prize. The amount of DNA we share with C. elegans was one of the first genomic surprises.
 
Nematode lifestyles. Meanwhile, the economically unimportant, non-destructive free-living nematodes of freshwater, which are the only ones you'll be seeing in the aquarium, are often ignored by the scientific community: no funding there! Free-living aquatic nematodes range from harmless to actively beneficial. Nematodes are an essential part of the community that breaks down detritus. Microscopic ones are ingesting bacteria and some fungi and other single-cell organisms, all creatures that would make your water cloudy if they got out of control. They feed in detritus and biofilm, where they specialize eating algae, bacteria or piercing the threadlike hyphae of fungi. Other nematodes live on or in plant tissues, or a few can be minute predators, mostly of other nematodes, but the largest predatory ones attack rotifers. 
 
Most nematodes produce hardy, resistant eggs, which hatch as miniatures of the adults. A few bear live young. Nematodes lack a free-swimming larval stage.
 
Nematodes in the filter. The first time you notice them may be when you have turned off your filter to make some adjustment. Then when the filter is turned back on, a zillion nematodes that have been dislodged from their comfortable niche are blown squirming into the water column, where your Neon Tetras make a meal of them! Fear not! all is well. Nematodes in the filter are grazing on bacteria there, and their predations are important for stimulating bacterial activity; three authors in 1997 found that there was consistently higher bacterial activity in sediment, such as your filter gunk, when nematodes were feeding on a bacterial community. (See an abstract of the article "The effects of nematodes on bacterial activity and abundance in freshwater sediment," originally published in Oecologia, vol. 112.) If nematodes make you nervous, keep repeating, "If I can see it in the water, it's not a parasite."
 
Nematodes form part of the early food for fish fry. Two kinds of nematodes I culture specifically for fry food are vinegar "eelworms" and microworms.
 
Identification of nematodes. But what is that nematode thrashing in the water, waving from a shred of algae or squirming among the gravel grains? Sometimes an identification allays our anxieties. If you have a powerful 20x hand-held magnifier or a simple microscope, you may be able to inspect a nematode well enough to want to identify it, at least to the family level. Identifying nematodes to the genus level is work for specialists (helmithologists).
 
Links. A clear crisp "Introduction to the Nematoda" is part of the UC Berkeley mega-site.  
 
 DNA research on nematodes is booming, generating a lot of the buzz at Paul de Ley's website "Wormwood: a resource for nematode phylogeny". Explore that site. There's plenty there for the non-scientist too: it takes an electron microscope to show you how nematodes partly inspired the huge Sandworms in the cult classic movie Dune.
 
Paul de Ley's "Bizarre Worm Bazaar"   has the Web's best weird nematode portraits.
 
An introduction to nematodes is maintained by the University of Nebraska, where people are naturally most concerned with the nematodes that are parasites of plants and insects. There's also a fantastic "Illustrated Key to Nematodes Found in Fresh Water", to help you identify the nematodes in your tank, if you're curious. First, though, you'll have to catch one with a pipet and get it onto a glass slip for the microscope.