Water flea and paramecium. Polarised light micrograph of a Chydorus sp. water flea (centre) amongst Paramecium bursaria (green). Water fleas are small crustaceans, commonly found in fresh water. They are filter feeders that ingest algae, protozoa or organic matter, and are a constituent of plankton. Paramecium are aquatic, unicellular ciliate protozoans which feed mainly on bacteria and algae. Magnification: x200 when printed at 10 centimetres wide.