Many advanced hobbiests and veterinarians have been asking me to put up some video's I've been compiling to tell the differences between the common ciliated protozoans we see attacking our fish. Aquarium fish can suffer from a variety of parasitic protozoans but sometimes it's tough to tell these "bugs" from bacteria "grossly" by just looking at the fish. It's also often important to know which "bug" we are dealing with. Using a microscope slide to scrape the side of a fish (if you can catch it) then mounting that onto a slide and looking under a microscope is the best way to diagnose disease affecting the outside of fish. Cryptocaryon irritans, Brooklynella sp., and Uronema sp. are similar  but hopefully these videos you can see key features which discern them.

Cryptocaryon: https://www.youtube.com/watch?v=DT_YMn2kkSI

Brooklynella: https://www.youtube.com/watch?v=c8Nl9bvUYg8

Uronema: https://www.youtube.com/watch?v=eM-Ok5-8JU8