Fish parasites can include both internal and external parasites. By definition, a parasite is an organism that lives on another organism; often the parasite benefits while the host organism suffers in some way. Fish parasites are often introduced into home aquariums when new fish are 
brought in and directly added to the existing tank without any quarantine period. Some common freshwater fish parasites include Whitespot (also called Ich), Argulus (fish lice), Flukes, Anchor Worms, and Leeches.
Whitespot, also commonly referred to as Ich, is actually caused by protozoa known as Ichthyophthirius. This is one of the most common problems in home aquariums, but can be easily dealt with if detected early. Fish afflicted with Ich have white spots on their bodies and fins, clamped fins, and visible trouble breathing.
Argulus, also referred to as fish lice, can be detected by the visible parasites on the body of the fish. These parasites are around ¼” in diameter; fish afflicted with fish lice often scrape against objects in the tank. The fish may also have clamped fins, and discolored or inflamed areas where the lice have bitten.
Flukes are flatworms that are around 1 mm in length; while the worms themselves usually can’t be seen, the symptoms are often obvious. Fish affected by flukes have very rapid gill movement, and often scrape against objects in the tank. Infestations in the gills or fins of the fish can actually eat away at them. Fish are often pale in color, with mucus covering their bodies or gills.
Anchor worms are small crustaceans that burrow into the fish, lay their eggs, then die. Fish afflicted by them often have inflames areas with whitish-green threads
hanging out.
Leeches are often apparent on the outside of the fish they infest. They may attach themselves to the bodies, gills, or fins of the fish, and often curl up into a heart shape. Most often, leeches infest an aquarium after being introduced on live plants or snails that are added to the tank.
Fish parasites can affect your fish either internally or externally. Some external fish parasites can be seen with the naked eye, while others are microscopic. Parasites should be diagnosed and treated promptly, to minimize adverse effects on your fish. Being sure to only buy fish from reputable dealers and always observing recommended quarantine periods can help minimize your likelihood of having to deal with fish parasites.
