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 from many different tissues and may cause a broad array of health conditions. It is critical that all growths or tumors seen on your fish are diagnosed by a qualified veterinarian as soon as they are first noticed. Waiting on treatment can seriously impact the prognosis of your pet fish.

Causes of Tumors in Pet Fish

A tumor is an abnormal swelling or bump in the body tissues. Tumors can be benign, meaning that they do not spread but grow in a single location, or malignant, meaning they may spread to other tissues throughout the body. Malignant tumor cells grow in an uncontrolled way and can invade nearby tissues and spread to other parts of the body through the blood and lymph system.

Cancer, which is Latin for crab, is the term used to describe malignant tumors that spread throughout the surrounding tissues, extending into healthy tissues like the legs of a crab. Cancer that spreads from where it started to a distant part of the body is called metastatic cancer.

All tumors occur when cell division goes awry. Instead of the cell's natural defense mechanism terminating the rogue cell, the abnormal 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 such as Iridovirus that can present abnormal growths similarly to tumors that need to be considered. Iridovirus causes lymphocystis lesions, which occur as a white cutaneous mass on the fins of fish.

Examples of Types of Tumors in Pet Fish


Neurofibromas are most commonly associated with goldfish. These nerve sheath tumors cause localized skin and fin 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, increasing their drag while swimming. These tumors penetrate deeply into the skin tissues and can have extensive vascularization, impacting blood flow. Surgical removal under anesthesia is often curative, especially when they are on the fins. Treatment with medications 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 Corydoras catfish species. They can arise from any type of pigment cell, each with its own unique name. For example, melanophoromas arise from the black melanin-containing cells called melanophores and iridophoromas arise from crystalized or sparkling pigment cells called iridophores.

In koi, chromatophoromas often occur on the dorsal surface of the skin and 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. There may be signs of the body swelling, scales protruding or abnormal swimming in the affected fish.

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, making a diagnosis using radiology or ultrasound, a fish can undergo surgery to remove the tumor. If not, the tumor will continue to grow and eventually interfere with the normal organ functions. Pressure from the tumor on the intestines and liver can lead to the fish not eating, or death from tissue necrosis.

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 to determine its source and best treatment. 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, surgical excision is used to remove the tumor for both diagnostic testing as well as treatment.

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

Both radiographs and ultrasound, as well as biopsy/cytology, may be utilized to make a diagnosis and determine the effects of the tumor on your fish's health.


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’s concerns for cost.

External tumors are frequently treated with surgical excision and 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. The fish is anesthetized and removed from the water, keeping it wet with moistened gauze placed on its skin. A water pump flushes anesthesia solution through the fish’s mouth and across its gills to keep it oxygenated and anesthetized. Under these conditions, a fish can be removed from the water for 30-60 minutes while surgery is performed to remove an internal tumor. After suturing the incision, the fish is returned to clean water where it will gradually awaken from the anesthesia.

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.

Currently, we don't know enough about the causes of tumors in pet fish to give owners a specific list of prevention strategies. Proper water quality and good nutrition go a long way in preventing diseases in fish. 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 have a long and healthy life.

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.
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