Dero worms are a very small type of aquatic worm sold as "Microfex" worms. Dero is apparently a genus that they belong to (or did). They're tiny. They're very slow to get going, as I mentioned, and the only way I've been successful is to polyculture them with Daphnia. You drop in algae wafers or some such, and the worms eat that, produce waste, which in turn feeds bacteria and the Daphnia. They're fully aquatic, with gills. They're covered in cilia, and the cilia look like little hooks -- I'm not sure if the fish are bothered by that or not.
They appear sporadically on Aquabid. Not often though. Because they're hard to and slow to culture, I don't think they're very popular.