Tumors and Neoplasia in Koi: What to Do When You Find a Growth

By KoiQuanta Editorial Team|Updated Mar 2026

Koi papilloma viruses cause benign wart-like growths that are entirely harmless, while true malignant neoplasms require surgical intervention. This distinction determines everything about how you should respond to finding a growth on your koi - and it's not always a distinction you can make by looking at the growth.

KoiQuanta's photo-documented growth tracking creates a time-stamped size record of koi tumors that supports accurate vet assessment remotely.

TL;DR

These are caused by Cyprinid herpesvirus 1 (CyHV-1), a different virus from Koi Herpesvirus (CyHV-3).
When you find a growth on a koi, immediately: 1.
Log the discovery date in KoiQuanta's fish profile 2.
Photograph the growth from consistent angles 3.
Measure the growth if possible (mm) and record 4.
Procedures are performed under anesthesia (typically MS-222 or clove oil) with the fish maintained moist throughout.

Types of Growths in Koi

Koi develop several distinct categories of external and internal growths, ranging from clinically insignificant to life-threatening:

Benign Viral Papillomas (Carp Pox)

The most common growth seen in koi is carp pox - smooth, waxy, white to pink raised plaques or droplet-like growths that appear on the skin, fins, and lips. These are caused by Cyprinid herpesvirus 1 (CyHV-1), a different virus from Koi Herpesvirus (CyHV-3).

Carp pox growths are:

Waxy, smooth-surfaced, candle-wax appearance
White, pale pink, or gray-white in color
Painless and non-invasive
More common in cool water - often regress spontaneously in warm summer temperatures
Not a health threat to the fish or to other fish in the pond

No treatment is needed or effective for carp pox. Growths may recur seasonally in affected fish.

Benign Cysts and Granulomas

Various non-neoplastic cysts develop in koi:

Lymphocystis - a viral disease causing large, cauliflower-like nodular growths on fins and skin. The growths are individual cells (fibroblasts) that have become massively enlarged by the virus. Benign and usually self-resolving.
Parasitic cysts - Myxozoa and other internal parasites can form visible cysts in skin and muscle tissue
Epidermal papillomas - proliferative skin growths that may have a viral component; generally benign

True Neoplasia (Tumors)

Genuine neoplastic growths - true tumors - do occur in koi:

Benign neoplasms (grow locally, don't invade or spread):

Lipomas (fat cell tumors) - smooth, soft, subcutaneous lumps, often mobile under the skin
Fibromas - firm, smooth connective tissue tumors
Myxomas - gelatinous soft tissue tumors

Malignant neoplasms (invasive, may metastasize):

Carcinomas affecting various organs
Sarcomas of connective tissue
Melanomas affecting pigment cells

Malignant tumors grow faster, are less well-defined at their margins, may ulcerate through the skin, and can affect fish behavior and condition as they consume resources.

Internal Organ Tumors

Koi can develop tumors of internal organs - gonadal tumors (particularly in older fish), kidney tumors, liver tumors - that are not externally visible but may cause abdominal distension, buoyancy changes, or wasting.

Assessing a Growth

Size and growth rate are the primary assessment criteria. A growth that has been stable in size for months is less concerning than one that has doubled in size in two weeks.

Texture and surface provide clues. Smooth, well-defined, waxy surfaces suggest benign carp pox or cyst. Irregular, ulcerated, or friable surfaces suggest malignancy. A growth with central ulceration suggests invasive growth through overlying skin.

Effect on the fish matters. Does the growth affect the fish's mobility, feeding, or buoyancy? A large internal tumor causing significant distension is more urgent than a small surface lesion that hasn't changed in a year.

Location affects urgency. A growth near the eye, mouth, or gill area can become functionally significant at a smaller size than a growth on the lateral flank.

KoiQuanta Photo-Documented Growth Tracking

This is where KoiQuanta's photo-documented size tracker provides direct value. When you find a growth on a koi, immediately:

Log the discovery date in KoiQuanta's fish profile
Photograph the growth from consistent angles
Measure the growth if possible (mm) and record
Note any associated symptoms (ulceration, behavioral change, appetite change)

Monthly updates with new photographs and measurements create the time-stamped record that shows growth rate clearly. A vet reviewing a growth remotely via your KoiQuanta photos can assess size change, surface change, and overall progression - information that's critical for deciding whether surgical intervention is warranted.

Surgical Options for Koi Tumors

Koi surgery by an experienced fish veterinarian has become increasingly available in recent years. Procedures are performed under anesthesia (typically MS-222 or clove oil) with the fish maintained moist throughout.

Feasible surgical procedures:

Excision of accessible external growths (lipomas, fibromas, accessible carcinomas)
Tumor debulking when complete excision isn't possible
Gonadal tumor removal
Excision of large carp pox growths causing functional problems

Post-surgical care involves wound management (topical antibiotics, clean water), salt for osmoregulatory support, and monitoring for infection. Recovery from well-performed fish surgery can be excellent.

Surgical intervention isn't always appropriate or accessible. For truly benign growths that aren't causing functional problems, monitoring and documenting is more appropriate than intervention.

Your koi disease identification guide and koi disease treatment tracker provide the broader context for managing neoplastic disease documentation.

Frequently Asked Questions

Is a growth on my koi cancerous?

Not necessarily. The majority of external growths found on koi are benign - carp pox from CyHV-1 virus (smooth, waxy, white plaques), lymphocystis (cauliflower-like nodular growths), parasitic cysts, or benign neoplasms like lipomas and fibromas. True malignant neoplasms occur in koi but are less common than benign growths. Assessment based on growth rate, surface texture, location, and effect on the fish gives useful clues, but definitive determination requires either biopsy with histopathology or a qualified fish vet examination. KoiQuanta's photo-documented tracking over time helps vets assess progression without requiring repeated in-person examinations.

Can koi tumors be surgically removed?

Yes. Fish surgery by qualified aquatic veterinarians can safely excise many types of external and internal tumors under appropriate anesthesia. Lipomas, fibromas, accessible carcinomas, and large carp pox growths causing functional problems can all be surgically addressed. The feasibility depends on the tumor's size, location, and the fish's overall health status. Post-surgical care is important for successful recovery. Not all tumors warrant surgery - stable, small, benign-appearing growths that aren't causing functional problems may be better managed by monitoring than by intervention.

How do I know if my koi tumor is growing?

The only reliable way to know if a growth is changing is to measure and document it consistently over time. KoiQuanta's growth tracking feature allows you to record measurements and upload photographs at regular intervals - monthly is appropriate for most stable growths, weekly if rapid change is suspected. By comparing the measurement and photograph from this month to those from three months ago, you can see clearly whether growth is occurring. A growth that doubles in size over two months is very different from one that has been the same size for a year. Share this documented progression with your vet to support treatment decisions.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association