Do you have trouble slowing down the protozoa? Swimming around by thousands of ciliate, they are surely moving fast. You have hard time getting them stay in the field of view not mentioning focus and switch to higher magnification. A 2-3% solution of Methyl Cellulose will slow down protozoan for better observation. If you don’t have it, you can use some cottons to confine them. How about the natural deterant, black hair algea? I have plenty of thse in my aquarium.
Microscope Preparation Starter Kits
Books for Protozoa identification