Parasites are common causes of disease in aquarium and pond fish.  Often, parasitic problems are secondary to an underlying stressor (poor water quality, shipping, trauma etc).  Click on the below links for more information and pictures of various tropical fish parasites.

PROTOZOAL PARASITES

CRUSTACEAN PARASITES

MONOGENEAN PARASITES (Flukes)

NEMATODE PARASITES

CESTODE PARASITES