One of my current pond dips is very rich in microorganisms haven't seen in ages. Many Volvox, Micrasterias and Stentor. Here are a couple of Stentor clusters.
I just started shooting darkfield on my main scope after recently fabricating a set of stops that I wrote about in an earlier post about hydra. Even though the condenser diaphragm has to be open wider than with brightfield illumination, I can still stop it down a little.
I'm finding the downside of darkfield is much greater sensor noise on the camera because of the longer exposures required in darkfield microscopy.
Images made through a microscope. All subject types.