Hydra. Hydra are solitary polyps in the Cnidaria phylum, the same phylum as sea jellies, sea anemones and corals; hydra are almost the only cnidarians that have managed to invade freshwater. (Well, okay, there's a little medusa of temperate lakes called Craspedacusta.) Hydra are not parasites, they are little predators. Such little predators, it's true, that you may not even notice them until they catch enough baby brine shrimp to turn pinkish. Like many small translucent invertebrates, they take on the colors of their most recent meal. The kind of fishkeeper who jumps to conclusions can become quite convinced that the hydra came in somehow with the dried brine shrimp cysts, but you'll quickly realize that is impossible for these creatures that are limited to fresh water. In fact, to eliminate hydra from daphnia or moina cultures, the only dependable technique is to let the substrate dry out completely. The daphnids will regenerate from their resting cysts, but the hydra will be gone until they are re-introduced.
Hydra are strictly carnivores. Flake feed and other detritus don't tempt them at all, but they do ingest the ciliates and other minute organisms that uneaten flakes support. A 10x magnifier may show you their basal disc, attached to the tank glass. On a good day, at 10x I can just make out the cnidoblasts, which contain the unique nematocysts, closely dotted along the tentacles. These discharge stinging barbs into the hydra's minute prey.
Hydra have been known to get into a culture of Moina (miniature Daphnia) here at my place and cause the population to crash, and they are definitely dis-commended in fry tanks. Yet anti-Hydra measures will be very stressful to fish fry. I don't know how dangerous they'd be to baby guppies, but they can certainly catch and eat gourami fry. In turn, some grown gouramis will eat hydra, which seems like rough justice, doesn't it? But hydra don't transmit any diseases to fish. None.
Hydra multiply by budding off polyps and have amazing powers of regeneration, hence their scientific name. The original Hydra of Antiquity was a pre-Hellenic multi-headed water-dwelling snake god that was already ancient when Hercules battled it as one of his Labors. At each sucessful stroke of Hercules' sword, two heads grew where one had been. (The resourceful Hercules took a flaming torch and swiftly cauterized each cut in turn, which saved his ass.)
Most hydra species are colorless, like the patriotic Hydra americana. Green hydra, Hydra viridis, contain symbiotic single-cell algae, Chlorella, which thrive within vacuoles in the hydras' cells and give them a bright lime-green color. Though other Chlorella species take up residence inside freshwater protozoa and sponges in chilly lakes or turn the Spotted Salamander's egg masses green, H. viridis is probably the closest you'll come to witnessing a freshwater version of the symbiotic zooxanthellae that reefkeepers nurture in the tissues of their Tridachna clams. In bright light the algae derive CO2, which they need for photosynthesis, from the constant output of cellular respiration, and they scavenge the ammonia that's another cellular by-product. In exchange the algae give off oxygen. The hydra are too primitive to organize any respiration more ambitious than the cellular respiration of each individual cell, but in a way the symbiotic algae function as a substitute for a green hydra lung.
Hydra can be controlled, if you must, with
Formalin or with Clout or Fluke-Tabs, at
low dosages that won't stress grown fishes.
If you have no fish in the aquarium, sodium
metabisulfite (Na2S2O5) or "Campden tablets"are being
used to counter hydra by Killifish pro Tim
"Campden tablets" are better known
for removing extra chlorine from drinking
water or eliminating unwanted bacteria and
yeasts in brewing and winemaking, rendering
tannin extracts more soluble, etc. I'm just
reporting here, as I've never personally
gone on a hydra-exterminating campaign.
I. Sinclair describes his modestly successful attempts at eradicating hydra at http://www.xenotoca.clara.net/hydra.htm Perhaps you'll agree that he exaggerates, for sake of a better tale, the dangers hydra pose.
The pros at theKrib.com offer lots of hydra postings, with the widest imaginable array of anti-Hydra measures (maybe a sign that nothing really works).
Beautiful pix of hydras by Jan Parmentier are at the Microscopy-UK website.
In the 1740s Abraham Tremblay was collecting Hydra and other plankton from the garden ponds of Sorgvliet, near The Hague, to instruct his aristocratic pupils, and was cultivating them in glass jars, the direct precursors of glass aquaria. A 1744 engraving of Tremblay's hydra jar, and another one depicting Hercules and the Hydra as painted on a Greek vase, accompany some pretty advanced molecular biology at the website where Robert Fischer is "Exploring the diploblastic freshwater polyp Hydra".