Large Lump on Koi: Ulcer or Tumor?

Observation

Large Chagoi has a large lump on side. Water quality good, eating well. Photo shows raised, irregular mass.

Ulcer/Abscess: Bacterial infection (Aeromonas, Pseudomonas) often secondary to parasite damage or injury.
Fibroma/Tumor: Benign connective tissue growth; common in older koi.
Carp Pox: Viral (Herpesvirus cyprini); waxy, candle-drip lesions; usually harmless.
Parasite cyst: Encysted trematode or nematode.

Veterinary exam: Koi vet can biopsy (needle aspirate or excision under anesthesia) and send for histopathology.
If ulcer: Topical antiseptic (iodine), injectable antibiotics (e.g., enrofloxacin), improve water quality.
If tumor: Surgical removal if accessible; otherwise monitor.
Quarantine if infectious disease suspected.

Attempt to squeeze, lance, or treat blindly. Stress worsens condition.

Pond and Water Gardening: Spring Koi Deaths: Diagnosing Recurring Losses; Overwintering Large Koi in Minnesota: 4,000 Gal, 100+ Fish; Butterfly Koi Lethargic After New Koi Added; Recurring Red Rash on Koi in Overstocked Tank: Parasites, Bacteria, or Stress?; Sudden Large Koi Death in Heat: Oxygen, Chlorine, and Gill Damage; Wintering Koi in Minnesota: Why a Fish Bowl Failed & How to Do It Right; Building a Koi Pond: Depth Requirements and Planting Water Lilies; Building a Large Clay-Bottom Koi Pond in South Texas; Black-Eyed Koi Fry: Normal Variation or Cause for Concern?; Sick Koi Floating Upside Down: Egg-Bound or Dropsy?

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