Goldfish & Koi Carp junior fishing guide

Schyzocotyle acheilognathi, or the Asian tapeworm, is a long ribbon-like parasite that lives inside the intestines of carp and other freshwater fish. It attaches to the gut wall and steals the fish’s food, which can slow their growth, weaken them, and even block the intestine completely. Young fish are especially at risk, and heavy infections can cause serious health problems or death, so good fishery management and biosecurity are important to stop it spreading.

Carp pox is a common, mostly harmless virus that causes smooth white waxy patches on carp during winter and early spring. The lumps look worrying but rarely harm the fish and usually fade as the water warms up. Good water quality and low stress levels help keep the disease under control.

Koi Herpesvirus (KHV) is a serious virus that infects only carp and can cause large-scale deaths, especially in warm water. It leads to severe gill damage, breathing problems and “sleepy”, weak behaviour. The virus spreads easily on fish, nets, and water, and must be confirmed by laboratory testing. KHV is a Notifiable Disease and must be reported immediately.

Chilodonella is a fast-spreading parasite that attacks the skin and gills of freshwater fish. It can cause heavy mucus, grey patches, breathing problems, and even death if untreated. It thrives in poor water quality, overcrowded waters, and when fish are stressed. Good fishery management is the best way to prevent outbreaks.

Gyrodactylus sprostonae is a newly recognised gill fluke causing major carp deaths in the UK. It reproduces extremely quickly, attaches to the gills, and can cause severe breathing problems and mortality. It mainly affects carp and crucians and is considered a serious emerging parasite. Good fishery management and quick reporting are essential.

Ichthyobodo necator (Costia) is a rapidly spreading parasite that attacks a fish’s skin and gills. It causes lethargy, breathing problems, weight loss, and can be deadly—especially for young or stressed fish. It thrives in both cold and warm water, so good management and low-stress conditions are essential to prevent outbreaks.