Ostracods

Ostracods ("seed shrimp") are minute crustaceans, usually less than 1mm across, smaller even than Moina, the mini daphnia. In a flash, ostracods can completely withdraw into their hinged protective lima-bean-shaped carapaces or ostraca (Greek ostracon means "shell"), which are so hard that many small fishes can't eat them and so small bigger fishes can't be bothered. Their nauplius larvae are already protected with a hard shell. These shells, which they periodically outgrow and shed at molts,  fossilize abundantly, rendering ostracoda useful to palaeontologists as species that identify the ancient ecology of geological strata.
 
A pair of sensitive swimming and grasping antennae project at one end. Another pair is hidden inside the shell. If daphnia hop, ostracods scoot, using two pairs of legs to scrabble nimbly over detritus and grains of sand. They are harmless detritus feeders, usually bouncing among gravel grains, but you'll probably first notice them zipping around near the water surface.
 
Ostracods are so minute that their coiled sperm in their testis may be longer than they are. On the other hand, some species are wholly or seasonally parthenogenetic. 
 
Like branchipods or "seed shrimp", ostracod  populations survive droughts as dessicated cysts. They are just part of the wider community in a planted aquarium, and I don't discourage them, on the general principle that diversity means stability. In natural environments, dense populations of ostracods are usually a symptom of over-rich water; so, if ostracods are noticeably numerous, some detritus-vacuuming and water changes may be in order.
 
Links. The Micrographia site has a page of freshwater ostracod illustrations. Scanning electron microscopy identifies ostracods to the family level. The Ostracod Research Group linking biologists and palaeontologists offers a good introduction to ostracods. You'll find great color video stills there, and the Introduction to ostracods gives you basic anatomy. Their crunchy calcified cells fossilize nicely, so you can get a hint of their fossil history back to Cambrian or Ordovician times, no less. And there are some details of their sex life, or lack of it. Because if your sex life is getting you down, remember that there are certain lineages of parthenogenetic ostracods that haven't had a date since the Jurassic! Have a look at "Posters: Geographical parthogenesis in European non-marine ostracods." at that O.R.G. site. ...And if that's not enough,look at the website of IRGO- the International Research Group on Ostracoda.
Ostracods at Wikipedia.