Most of the protozoa and microbes shown here are much smaller than 500 microns, and are therefore considered truly microscopic, and can be viewed only with the aid of a microscope.
Rotifers, on the other hand, can grow as large as 500 microns (1/2 mm) and can probably be seen with the naked eye, or at least with a 10x magnifying glass. The Rotifer shown here, however, is probably smaller than 200 microns.
The fascinating Vorticella never exceeds 150 microns, so it requires a microcope for viewing. A 100x magnification would be sufficient to see its cilia swirling water and food particles into its huge mouth.
The interesting Actinosphaerium grows between 200 and 800 microns, the larger being visible with the naked eye. The ones pictured here were extremely small, certainly in the vicinity of 200 microns.
The water drop taken from the culture of dried leaves and grass contained a swarming mass of assorted microbes. Many were moving so quickly it was possible to capture them only as blurs. Most were as small as only 1 or 2 microns, some ciliates (locomotion using cilia) and some flagellates (locomotion by wiggling their bodies or their tails).
The largest creatures shown in these photographs, by far, are the Bloodworms. They can be seen easily with the naked eye, usually nestled securely in the pond filter media or in the bottom muck.