Gill flukes (Dactylogyrus) and skin flukes (Gyrodactylus) are common parasites that affect fishes. Like popular community fish, they are split between livebearers and egg layers, but that's where the similarities end.
Despite the given names, they are both likely to be found on the gills or skin and although often found in small numbers on healthy fish, large numbers can be very debilitating. Flukes are often the cause of damage that leads to bacterial secondary infections and, being microscopic, are hard to detect.
Like most parasites, controlling background waste levels and maintaining good water quality is important in reducing populations and ensuring fish have a healthy immune system. As flukes are often the root of ulcers and other secondary infections, treat for flukes before treating with antibacterial or antifungal medications. A number of treatments are available for flukes, those containing Praziquantel are the most effective.