Koi with a Bump: Tumor, Cyst, or Something Else?

Finding a lump on your koi can be alarming. While tumors are possible, many growths are benign or treatable. Here's how to assess the situation and decide on next steps.

Common Causes of Bumps on Koi

• Cysts (fluid-filled): Often benign, may resolve or persist.
• Lipomas (fatty tumors): Soft, movable, usually harmless.
• Parasitic granulomas: Encapsulated parasites like Lernaea (anchor worm) or flukes.
• Bacterial abscesses: From infections like Aeromonas or fish tuberculosis (mycobacteria).
• Viral growths: Carp pox (herpesvirus) causes waxy, white-to-pink nodules, often seasonal.
• True neoplasms: Rare but possible; may be malignant.

Key Questions to Narrow It Down

1. Color? White/pink = carp pox or abscess; red/inflamed = bacterial; same as skin = lipoma/cyst.
2. Size and growth rate? Fast-growing = more concerning.
3. Texture? Soft/fluctuant = fluid; firm = solid tissue.
4. Other fish affected? If only one fish and no new additions, contagious causes are less likely.

Recommended First Steps

1. Observe and photograph: Track changes weekly.
2. Test water: Rule out environmental stressors (ammonia, pH, low oxygen).
3. Conservative treatment: Pond salt (0.05–0.1%), MelaFix (natural antibacterial), and if bacterial suspected, a broad-spectrum antibiotic like erythromycin (Maracyn) or a vet-prescribed injectable.
4. Avoid harsh chemicals unless a parasite is confirmed—many medications stress koi further.

When to Seek Expert Help

If the bump grows rapidly, ulcerates, or the fish shows systemic signs (lethargy, isolation, loss of appetite), consult a koi veterinarian or experienced koi club member. A biopsy or scrape may be needed for definitive diagnosis.

Many koi live years with benign growths. Don't panic—start with observation and supportive care.