Flukes are classified as trematodes and are among the largest microscopic parasites. They are worm-shaped with a mouth at one end and a suction cup-shaped foot at the other end. The foot has several claw-like anchors called haptens. They use these haptens to puncture the fish’s skin so they can hold on while feeding on the slime coat. As they puncture the fish’s protective skin layer, they often introduce Aeromonas or Pseudomonas bacteria, thereby causing an outbreak of bacterial sores on the fish. In large numbers, they can kill fish even without the bacteria issue.