I took this from a website, www . reefaquarium . com /2014/flukes-marine-fish/
There are two common types of flukes, the ones that infect the skin of the fish (dactylogyrus trematodes), and the second that will infect both the skin and gills of the fish (monogenenean trematodes). Both can be very hard (if not almost impossible) to visually see on the fish until very advanced stages. While attached to the fish, they will feed on the tissues they are attached to. At some point they will start releasing eggs which will fall and lay dormant within the aquarium for about a week. They will hatch , releasing a free floating / swimming larva. This larva will survive for two or three days without attaching to a host before dieing. Typically the larvae will attach itself to either the gills or skin of the fish. Once attached, the larva will develop into a worm and releasing eggs continuing the life cycle. This final stage can last for about a week, maybe a little longer