Not all the plankton are utterly invisible. The largest free-swimming rotifers are just discernible, if you set a glass of tank water into a sunny window and peer into it with a magnifying glass. They're too small to count as invertebrate visitors, but they're a major component of the food web.
Rotifers are the most abundant and cosmopolitan of freshwater zooplankton. Eventually they will turn up, even in an unplanted aquarium, partly because they can resist desiccation by secreting a protective gel envelope and blowing in from outdoors in a spore-like fashion. Rotifers can survive freezing as well as drying, in a kind of suspended animation, which has enabled them to be distributed, windblown, from deserts to the poles, wherever there is occasionally enough free water that lasts long enough for rotifers to hatch, feed and reproduce. Which isn't very long: tropical water temperatures speed up the reproductive cycle of rotifers, right up to 90-91°F (32-33°C), above which they're increasingly stressed.
Rotifer means "wheel-bearer," because a rotifer's mouth is surrounded by many cilia, moving too fast to be individually distinguishable but working in coordination to whirl suspended particles of food into the mouth. So rotifers count as "filter feeders." In fresh water, we don't generally have many larger, more obvious filter-feeders, which are so characteristic of coral reef systems, unless we happen to keep freshwater mussels or Atyid shrimp.
Though rotifers are multicellular creatures, they are on the same scale as ciliates: they average 0.1 to 0.5 mm long, the largest of them just visible with a 10x magnifier. By the way, do you have a 10x folding loupe magnifier? Indispensible for peering at your gravel grains.
Though rotifers are universally distributed in the freshwater plankton, each ecotope supports a limited range of species that are particularly well-suited to that particular environment. In your aquarium, you'd expect to find many rotifers, but of only a few species.
There are few kinds of marine rotifers, perhaps fifty species, though lots of them, enough to raise Clownfish fry; but in fresh water rotifers are among the primary grazers. They subsist on bacteria and on the various photosynthesizing organisms. So planktonic rotifers join with the ciliates in keeping aquarium water from turning green. Sessile rotifers attach with a sticky foot to plant surfaces (making them literally "littoral"), or in the interstices of the biofilm, and on flocs of humus and grainy sediment. Rotifers are colorless, but a recent meal can tint them rusty brown or green. In turn rotifers form a food for fish fry.
The 2000 species described so far can't be more than a fraction of the rotifers that exist. Rotifer species run a broad gamut of lifestyles. Rotifers provide a major food source for fish larvae, but they also participate in the decomposition cycle. Proales species colonize the outer shells of daphnids and can settle quickly onto snail eggmasses or hitch a ride on the globular colonies of Volvox. Free-living rotifers feed on the bacteria that coat suspended organic floc. Some prey on single-celled protists and even on other rotifers. But Albertia is a parasite in the digestive tract of annelid worms (though harmless to fishes).
Link. A good brief not-too-technical introduction to the biology of rotifers is Roy Winsby's article "Rotifers and how to find them" archived at the Microscopy website.