This disease normally occurs with large female Koi. One belief is that this is a malignant tumor in the genital area. Another is it can be caused by the feeding of oxidized high protein food. More than 20 years ago, when Koi food was not available, a high protien food developed for other fish was used. It was noticed that some Koi had problems with large stomachs and they eventually died. When good quality Koi food did become available, a reduction in stomach tumors was seen.

It is very difficult to treat a Koi with a stomach tumor although some adventurous Koi keepers have tried surgical removal and been successful. If attempted at an early stage in the tumor's growth this method is about the only one available.

*Information on this page is an excerpt from "KOI" by Grant Fujita, available at the AKCA online bookstore.