Almost all fish are prone to parasite infestation. Most parasites do not cause much harm to the host when present in small numbers. However, they grow and reproduce quickly under ideal living conditions. When the population grows beyond a certain limit, it can be life-threatening. Parasites can commonly transmit to another organism quite easily. So, an infected fish needs to be isolated and treated as soon as possible. Indications of Infestation There are numerous signs that indicate the presence of parasites. They are:

• Presence of worms or leeches on the body of the fish.
• Presence of ulcers and holes on the body of the fish. This sign mostly indicates the presence of flukes.
• The infested fish's eyes are cloudy, or contain white patches.
• The affected individual often appears to gasp for air, and rubs its body against some object.
• When fish have internal parasites, they show a marked loss of appetite, erratic swimming, or torpidity.
• The area around the fish's tail becomes red, irritated.

Common Culprits Argulus or Fish Louse: Argulus is a flat crustacean. It attaches itself to the fish's body. It causes a lot of discomfort to the host, which develops clumped fins, inflamed body parts, and becomes restless. Anchor Worm: The fish hosting anchor worms, also called Lernaea, is often found to scrape its body against objects, and develops an inflamed area at the point of attachment. The worm hangs out its whitish-green thread-like body from the fish's skin. Anchor worms usually infest goldfish and other related carps. Ergasilus: Like anchor worms, Ergasilus makes the host fish scrape its body against some object and extends its whitish-green threads hanging from the fish's body. It mostly borrows into the gills. Chilodonella: Chilodonella is a ciliated protozoa, which causes the host fish to secrete excessive mucus. The host fish is often seen to flash and show irritation. When the number of Chilodonella becomes more than 10 in the host fish, it may die. The presence of scrapings of mucus and gill filaments under a light microscope indicates the infestation of Chilodonella in a fish. Cryptobia: Cryptobia is a flagellated protozoan, mostly found in the stomach of the host fish. It may take shelter in other organs of the fish as well. The infested fish may become thin, lethargic, and often develops dark skin pigmentation. Myxozoa: Myxozoa normally infest native and pond-reared fish populations. On infestation, it creates relatively less discomfort to the host. However, heavy infestation may be fatal. Coccidia: Coccidia is one of the common parasites in freshwater fish. It commonly affects the intestine, liver, swim bladder, and reproductive structures. Hexamita: Hexamita is usually seen in tropical fish. It is an unicellular organism, and infests fish like discus, angelfish, oscars, etc. Fish infested with Hexamita show certain specific signs and symptoms, such as loss of appetite and uneven swimming. They also develop holes around their head. Isopod: Isopods have sharp, nasty claws. They eat the tongue of the fish and replace the organ. If you breed fish in an aquarium, there is every likelihood that they may get infested with parasites. If it is so, you must take some necessary actions. This will save the healthy fish, and the aquarium as a whole, from getting disease-infested. The infected fish needs to be treated for the parasite. The entire content of the aquarium needs to be changed as it may contain eggs or larva of the parasite. To keep yourself on the safe side, call a professional for help.