Water mites (Hydrachnidia)

Mites: most of the arthropods living in freshwater are crustacea; the mites aren't crustaceans, however, but cousins of spiders. Mites are everywhere of course, the most specialized and morphologically diverse of all arachnids, comprising as many as half a million species, colonizing environments that range from your carpet and your eyelashes to the Antarctic wastes. Though many mites are parasites, you'll be relieved to know they don't parasitize fishes or amphibians. Few mites get as big as one millimeter, many are microscopic; I've found almost microscopic mites infesting old microworm cultures, and they're a common pest if you're culturing Grindal worms or whiteworms.
 
Several groups of mites have specialized for life in fresh water. Water mites may live near the surface film or among the plants or in detritus or among the gravel grains. They are astonishingly diverse and successful in natural ecotopes: predators and herbivores, eating detritus or specializing on fungi, parasitizing insects in their adult phase or just hitching rides. They don't hurt fish. You might barely notice the water mites, and they may be beneath the notice of the fishes too. The very smallest fish will eat them, though. But some of the bright-colored water mites are distasteful; they advertise their toxicity with bright warning colors. So it's likely that the mites you notice in the aquarium are the gaudy and mildly toxic ones, because the fish have already selected the edible ones for snacks. How's that for a demonstration of the effects that the pressures of predation bring to bear upon populations in an ecosystem, being worked out under your very eyes, right in your home aquarium?
 
Mites have resting stages between their life stages, or instars, a trait that has made them very resilient to changing conditions. Some can even survive when their body of water completely dries out. And the three instars— larvae, deuteronymphs and adults— each specialize in a different food resource. So the various instars of a species aren't in competition, another trait that has helped make them successful.
 
Links. A good brief, slightly technical account of water mites (and of collembolans) is at the "Freshwater Benthic Ecology and Aquatic Entomology Homepage". There's an illustrated introduction to the mites of North America at the website of the Ecological Monitoring and Assesssment Network  (Canada). The case study on the habitats and communities of water mites gives you a sense of the richness of freshwater communities, if you can penetrate the slightly stiff professional vocabulary.