Zooplankton: protists

The zooplankton, or "animal"-like organisms are a diverse group of minute, weakly-swimming organisms defined by their small size and their function in the energy system, rather than by any shared taxonomic affiliations. Many of the zooplankton are suspension feeders, straining out the phytoplankton and keeping your water clear. They may be single-celled organisms like flagellates or ciliates, or multicellular animals, such as microscopic nematodes or rotifers, or the small crustaceans, such as copepods and daphnids. Physiologically they may have structures that help them maintain buoyancy . Asexual reproduction helps them maintain large populations.
 
They are adapted to drift with the current (these are pelagic, a word that originally referred to the open sea), or to make a living attached to vegetation or other surfaces (these are littoral, a word that originally referred to the shoreline). Similar animals that carry on a sedentary existence attached to the silt and detritus of the substrate are referred to as benthic. The zooplankton in your aquarium, which take advantage of these three kinds of watery environments — pelagic, littoral or benthic — are very largely composed of three major groups of organisms: single-celled protists, rotifers, and crustacea. As we look in turn at each of these main groups, we are also working upwards through the food web.
 
Protists. Protists include the whole range of single-celled eukaryotes. Protists don't include prokaryotic bacteria and cyanobacteria on the one hand, and on the other they exclude all multicellular animals, even the simplest hydra or smallest rotifer.
 
Links. There's a general zooplankton site maintained by Southwest Missouri State University. It has a page devoted to identifying freshwater plankton.  University of British Columbia also has a visual compendium of freshwater plankton.