They are small water crustaceans also called the Cladocera, around 620 species have been recognized now, they are flea looking and look like small, transparent disks in the aquarium.
They are a parasite that may transfer sickness to your fish. It is hard to get rid of their eggs because of egg resting, it means their eggs can stop developing for a bit to endure harsh conditions. They swim in a tank, then attach themselves to a fish. If you see transparent disks on your fish around the gills, bring it to the vet!