Daphnia is a genus of small small aquatic crustaceans often called water fleas.
Several species including Daphnia magna are used in the aquarium hobby as life food.
Daphnia can often be collected in local ponds and water ways. Aquarist sometimes culture Daphnia in outdoor ponds in their backyard. Daphnia can also be cultured indoors, keeping them in an aquarium and feeding them a supply of green water or yeast.
Making a suitable live food for smaller adult fish, Daphnia is often cultured by killifish owners.Video - Daphnia - A View Under a Microscope