There are generally two types of fish parasites: ectoparasites, which are found on the external surfaces of the fish (skin, fins and gills) and endoparasites are typically found on the internal organs and tissues. Parasites can include, leeches, lice and anchor worms, which can be seen with the naked eye. Other indications are swelling or redness on certain surfaces of the fish, or the fish's tendency to rub or scrape itself against other surfaces.

There are also many parasites which cannot be seen with the naked eye. These can include the many types of protozoan parasites, which are single celled organisms, and are usually free living in the fish's environment. Protozoanic parasites are also among the easiest to identify and control. Signs of your fish having this type of parasitic problem may be indicated by "itch" or "white spot disease", gills that are swollen or covered with thick mucus, or excessive mucus creation in the fish. Even bulging eyes can be an indicator of parasitic activity.

Other signs that the cause of your fish losing weight is the result of parasitic activity in your fish, can include lethargy and heavy breathing, which may be seen in a fish with parasites affecting its gills. Often a mucus or gill sample can be taken for microscopic identification of parasites.

Another cause of fish losing weight can be bacterial infection. Since it is impossible to accurately diagnose bacterial disease in fish merely by sight, it is important that other tests be performed on the fish, such as mucus sample testing.

Though it may seem obvious, weight loss in your fish may be a simple result of it not liking the type of food you are using, thus the importance of knowing your breed of fish, and ensuring it gets the proper nutrients.

Since the cause of your fish losing weight can be as simple of they type of food you are providing, or more serious as parasitic or bacterial infection, early diagnosis and treatment is important.