Tumors in Pet Fish

Neurofibroma on the dorsal ridge of a goldfish

John Smith/Flickr

Unfortunately fish are not immune to cancerous tumors. They can arise as a result of many different tissues and cause a broad array of health conditions. It is critical that all potential growths or tumors are diagnosed by a qualified veterinarian as soon as they are first noted. Waiting on treatment can seriously impact the prognosis of your pet fish.

Causes of Tumors in Pet Fish

Like all other cancers, tumors occur when cell division goes awry. Instead of the cell's natural defense mechanism kicking in and terminating the rogue cell, the cell is allowed to replicate. Multiple replications cause a tumor to form. The rate of tumor growth depends on the type of cell and its access to nutrients.

Tumors can occur in all tissue types, but fish are more prone to developing tumors in their skin and reproductive organs. There are also viruses that can present similarly to tumors that need to be considered.

Examples of Types of Tumors in Pet Fish


Neurofibromas are most commonly associated with goldfish. These nerve sheath tumors cause localized lumps that can grow very large, fall off, and then regrow.

They are benign tumors and do not cause the fish any distress other than temporary hydrodynamic resistance, impacting on their swimming flow. These tumors penetrate deeply into the skin tissues and can have extensive vascularization, impacting on blood flow. Treatment is not recommended due to limited success.

Goldfish with neurofibroma on dorsal rigde
Dr. Jessie Sanders


These pigment cell tumors are seen in koi, butterfly fish and Corydora catfish species. They can arise from any pigment cell type, each with their own unique name. For example, melanophoromas arise from melanin-containing cells and iridophoromas arise from crystalized or sparkling pigment cells.

In koi, chromatophoromas often occur on the dorsal surface of the conjunctiva. These are more common in ponds that are exposed to lots of sunlight. Mild cases have been successfully treated with cryotherapy or the application of liquid nitrogen.

Koi with chromatophoroma
Dr. Jessie Sanders

Gonadal sarcoma

This is a common type of reproductive tumor, especially in koi where it is often mistaken for "egg binding." It is more common in older female koi and can be very hard to see externally.

It is usually only after the tumor is sufficiently large that the owner notices a change in their fish's appearance. If caught early enough, using ultrasound, a fish can undergo surgery to remove the tumor. If not, the tumor will eventually crush the fish from the inside out.

Ultrasound picture of gonadal sarcoma
Ultrasound picture of gonadal sarcoma in a koi. Dr. Jessie Sanders

Diagnostic Processes

Depending on the location of the tumor, your veterinarian can use multiple diagnostic techniques. For external tumors, a simple biopsy or cytological sample is used, collecting a few cells of the tumor. These samples will be processed and sent to a specialized laboratory for further analysis. Sometimes, a large biopsy or surgical excision is used as both diagnostic and treatment.

For suspected internal tumors, your veterinarian may utilize ultrasound or radiographs to check out the internal structures of your fish. Radiographs are best at observing bones and swim bladder structure, but when used with contrast, such as oral or aboral barium, the entire GI can be visualized. Ultrasound is used to observe the structure of soft tissue components, such as internal organs.

Both radiographs, ultrasound and biopsy/cytology may be utilized to determine the effects of the tumor on your fish's livelihood.


Depending on the type of tumor, there are a variety of treatment options. Traditional cancer therapies, such as chemotherapy and radiation therapy , have not been utilized much due to limited access and owner cost concerns.

External tumors are frequently treated with surgical excision with secondary topical treatment, such as cryotherapy. With this treatment, your veterinarian will anesthetize your fish, apply a local anesthetic, surgically cut away the mass, and apply liquid nitrogen to the spot to deter further cell growth.

Internal tumors will require surgery. Yes, fish can undergo surgery, just like other animals. Although the procedure is slightly altered, many fish have undergone successful surgical treatment for various types of internal tumors.

How to Prevent Tumors in Pet Fish

There is a suspected genetic predisposition to certain cancer types. Fish bred for certain external appearances, such as koi with large bellies and moor goldfish with large eyes, may be unknowingly selecting for potential tumors.

As of right now, we just don't know enough about tumors in pet fish to give owners a specific list of prevention strategies. The best recommendation is to evaluate all your fishes daily, and if one starts to look or act odd, contact your aquatic veterinarian as soon as possible to ensure your fish a long and healthy life.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Okihiro MS. Chromatophoromas in Two Species of Hawaiian Butterflyfish, Chaetodon multicinctus and C. miliarisVet Pathol. 1988;25(6):422-431.

  2. Stevens BN, Vergneau-Grosset C, Rodriguez CO, et al. Treatment of a facial myxoma in a goldfish (Carassius auratus) with intralesional bleomycin chemotherapy and radiation therapy. Journal of Exotic Pet Medicine. 2017;26(4):283-289.

  3. Sladky KK, Clarke EO. Fish surgery: presurgical preparation and common surgical procedures. Vet Clin North Am Exot Anim Pract. 2016;19(1):55-76.

  4. Harms CA, Lewbart GA. Surgery in fish. Vet Clin North Am Exot Anim Pract. 2000;3(3):759-774.