Hemangioma and Hemangiosarcoma in Koi Fish: Blood Vessel Tumors and Bleeding Masses

Hemangiomas and hemangiosarcomas are tumors that arise from blood vessel cells. A hemangioma is considered benign, meaning it tends to stay more localized. A hemangiosarcoma is malignant, meaning it can invade nearby tissue and may spread internally. In koi, these tumors may appear as red, purple, black-red, or blood-filled masses on the skin, fins, mouth, or body wall. Some are smooth and dome-shaped. Others ulcerate and bleed easily.

For pet parents, the hardest part is that these masses can resemble trauma, infection, inflamed tissue, or other tumor types. A bleeding lump is not always cancer, and a non-bleeding lump is not always harmless. Fish neoplasia is recognized in veterinary medicine, and surgery is increasingly used in selected fish with external masses, but the exact tumor type usually cannot be confirmed by appearance alone.

Because koi live in water, even a small surface mass can become a bigger problem than it first appears. Repeated rubbing, poor water quality, or secondary infection may worsen tissue damage. If the mass is internal, signs may be more subtle at first and can include swelling, weakness, or reduced swimming performance.

In many koi, the exact cause is never fully identified. Fish tumors can develop for reasons similar to those seen in other animals, including spontaneous cell changes, genetic susceptibility, age-related risk, and chronic tissue irritation. Veterinary references on fish neoplasia also note that some tumors in fish species are associated with viral causes, while others appear genetically mediated.

That said, pet parents should know that poor pond care does not directly cause every tumor. A well-cared-for koi can still develop a mass. However, chronic inflammation, repeated trauma, ultraviolet exposure in shallow systems, and long-term environmental stress may contribute to tissue damage that makes skin and soft tissue problems harder to control.

Water quality still matters. Even if it did not start the tumor, poor water conditions can worsen bleeding, delay healing, and increase the risk of secondary bacterial infection. This is one reason your vet may recommend both tumor evaluation and a review of ammonia, nitrite, nitrate, pH, temperature, stocking density, and filtration.

Diagnosis starts with a hands-on fish exam and a close look at the mass. Your vet may ask when the lesion first appeared, whether it has changed size, and whether it bleeds after netting or feeding. Photos over time can be very helpful. Because fish tumors can mimic infection, cysts, granulomas, and traumatic wounds, appearance alone is not enough for a final answer.

Your vet may recommend sedation so the koi can be examined safely and with less stress. In fish medicine, MS-222 is the most commonly used anesthetic, and surgery is increasingly used for selected neoplastic conditions. Depending on the location of the mass, your vet may collect a fine-needle sample, perform a biopsy, or remove the mass and submit tissue for histopathology. Histopathology is the most reliable way to tell a benign hemangioma from a malignant hemangiosarcoma.

If your vet suspects internal spread or a deeper tumor, additional testing may include ultrasound, radiographs when practical, blood or water-quality review, and sometimes necropsy if the fish dies or humane euthanasia is chosen. These steps help your vet separate a localized surgical problem from a more advanced cancer process.

There is no guaranteed way to prevent blood vessel tumors in koi. Because the exact cause is often unclear, prevention focuses on reducing avoidable stressors and catching problems early. Good pond management supports skin health and may lower the chance that a small lesion becomes a larger bleeding wound.

Aim for stable water quality, appropriate stocking density, reliable filtration, and regular observation during feeding. Check your koi for new lumps, color changes, ulcers, or repeated rubbing against surfaces. Early photos can help your vet judge whether a mass is changing quickly.

Quarantine new fish, avoid rough netting, and correct sharp pond edges or abrasive décor that can traumatize a fragile mass. If your koi develops any persistent lump, especially one that is red, dark, or prone to bleeding, schedule a veterinary visit sooner rather than later. Early evaluation gives your vet more options, including conservative monitoring, biopsy, or surgical removal when appropriate.