Inside this folder are the visible invertebrates we treat as visitors. This isn't a biology text or anything, so the Skeptical Aquarist splits up invertebrates according to the uses we make of them; this is a thoroughly medieval organizing concept, isn't it? something out of an illuminated Bestiary manuscript. But non-vertebrates are everywhere in your aquarium, so it does make fishkeeping sense.
Largely microscopic invertebrates are essential components of the plankton and the biofilm. They play roles in the food web. We supplement the natural food web by culturing invertebrates, even microscopic ones, as live food. And other invertebrates only come to our notice as pathogens, when they become parasites or vectors of disease.
Of the freshwater invertebrates we might purchase, only among the snails and shrimps are there some species that make good long-term companions to freshwater fishes and plants. Neale Monks' survey of "Invertebrates for freshwater aquariums" includes a table with the pros (limited) and cons (numerous) of freshwater shrimps, crayfish, crabs, snails, and bivalves in the aquarium. He does not share the Skeptical Aquarist's welcoming view of Malayan Trumpet Snails, Melanoides tuberculata.
Links. The best portal to web information about invertebrates (and vertebrates too) is UCal Berkeley Museum of Paleontology's massive site.
At the major site devoted to amateur microscopists www.microscopy-uk.org.uk look through the e-zine Micscape or try "The smallest page on the Web" among "Pond Life" articles in the Magazine Library. Or try the "Pond Life Identification Kit" at the site— or just do a keyword search at the site for the particular organism you're interested in.
Many favorite microscopist subjects are the pondwater organisms that are equally part of the aquarium's plankton and biofilm. Some of the variety of lives in the biofilm is captured in many color micrographs with informative captions at John Walsh's beautiful site "Micrographia" geared to professional and amateur microscopists. The Specimen Galleries at Micrographia illustrate algae broadly speaking, bacteria and cyanobacteria, protozoans and minute aquatic crustaceans, etc. with some rich and informative text.
Nikon make microscopes as well as cameras. They have a microscopy site with a "Digital Movie Gallery." Select "Pond Life," where you can view digital micro-videos of many creatures you'll also find in the aquarium: aeolosomatid and naidid and nematode worms, flatworms, hydra, daphnids, gastrotrichs, rotifers, waterbears, etc. (You need to configure your browser with RealPlayer to view them.)
There's a web homepage for freshwater benthic ecology and aquatic entomology that is coordinated from Halifax N.S. by Prof. S. M. Mandaville. It's got condensed information about all the various invertebrate orders and families likely to be found in fresh water, and copious links to excellent scientific stuff about the world of the freshwater biofilm.