Paramecium and Frontonia protozoa, light microscopy footage. Paramecium (thin, clear) are unicellular ciliate protozoa. They inhabit fresh water, and feed mainly on bacteria and smaller protozoa. They range from about 50 to 350 micrometres in length. Frontonia (larger, green) are unicellular large freshwater ciliate protists. They are capable of ingesting large prey such as diatoms and filamentous algae. Several ingested diatoms and blue-green algae can be seen inside them.