The most common type of water flea, Daphnia ambigua, eats bacteria and algae, according to Fairfax County Public Schools. Microscopy-UK and Wikipedia further note that some species, including Polyphemus pediculus and Leptodora kindtii, are known for eating small animals, including other water fleas.
Encyclopædia Britannica explains that water fleas also feed by filtering small particles from the water. They are equipped with thoracic limbs designed for this purpose.
According to the Encyclopædia Britannica, water fleas are essential to the natural food chain. The nutrients water fleas consume from the water and their prey are ultimately transferred to their own predators, most often fish.