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Sunburn, tumors and injuries

By the koi.wiki editorial team · Updated

Key facts: Sunburn, tumors, lymphocystis-like growths and physical injuries are non-infectious or non-standard health problems seen in koi, distinct from the parasitic, bacterial and viral diseases covered elsewhere.[1]
Beyond parasites, bacteria and viruses, koi keepers may encounter sunburn, tumors, growths resembling lymphocystis, and traumatic injuries from handling, predators or pond hazards. Kodama Koi Farm, a US koi dealer, lists tumors, ulcers, dropsy and swim bladder disease among conditions keepers should be able to recognize in a sick koi.[1]Kodama Koi Farm's symptom table describes “cauliflower-like growths on fins and body” attributed to an iridovirus and labeled lymphocystis.[1] However, according to University of Florida IFAS Extension, true lymphocystis disease, caused by Lymphocystivirus (family Iridoviridae), does not affect cyprinids such as goldfish, koi or barbs, and is instead documented in more than 125 other marine and freshwater fish species.[2] This is a notable disagreement between sources on whether lymphocystis occurs in koi at all.

Growths and tumors

Kodama Koi Farm's symptom-and-disease reference lists tumors as a cause of positive buoyancy disorder, noting that treatment can include surgically removing tumors that have displaced the swim bladder.[1] The same source describes a case in its reader comments of an 18-year-old koi found dead with a ruptured abdomen and a mass of eggs, which the owner considered might be a tumor, bloating, or an egg-related issue, though no diagnosis was confirmed.[1]External wart-like or cauliflower-like growths are sometimes labeled lymphocystis by hobbyist sources.[1] According to UF/IFAS, lesions of true lymphocystis disease are clustered masses of greatly enlarged fibroblast cells, each potentially growing from about 0.1 mm to over 2 mm, that act as “virus factories” before bursting and releasing virus.[2] The same source states this disease is usually self-limiting in warmwater fish, typically clearing within a few weeks with no specific treatment, and that it does not occur in cyprinids including koi.[2] Because wart-like nodules can also be confused with parasites, fungal clumps, mucus tags, or skin cancers, UF/IFAS recommends diagnosis by a qualified fish health professional.[2]

Sunburn

Sunburn is a radiation burn of living tissue caused by overexposure to ultraviolet (UV) radiation, documented primarily in humans and other animals, with signs including reddening, swelling, blistering and peeling of affected tissue.[3] According to Kodama Koi Farm, koi pond plants such as water lilies, lotus and water hyacinth provide shade that helps regulate water temperature and reduces the risk of koi sunburn and stress.[4]In general biological terms, UV radiation damages nucleic acids and triggers inflammatory responses in exposed tissue, with reflective surfaces such as water increasing UV exposure.[3] Ultraviolet germicidal irradiation sources, including the UV-C lamps used in some pond clarifiers, work on a similar principle of damaging nucleic acids, though at shorter germicidal wavelengths than those mainly responsible for sunburn in animals.[5] Koi keepers interested in such equipment can refer to the article on UV clarifiers and sterilizers.

Physical injury and trauma

Physical trauma in koi can result from handling, predator encounters, aggression between fish, or accidents in the pond. Kodama Koi Farm notes that fish veterinarians assess wounds, physical injuries and other abnormalities arising from accidents, predator encounters, handling, or interactions with other fish, and that evaluating the extent of an injury helps determine the appropriate course of action while reducing the risk of further complications.[6]According to UF/IFAS, external trauma from spawning, aggression, parasites, or handling can facilitate infection and disease spread in fish generally, while crowding, shipping and other stressors can trigger outbreaks.[2] Readers concerned about pond predators that can injure koi may consult the articles on herons and mammals that prey on koi.Injuries and abnormalities affecting buoyancy are among the physical conditions keepers may observe. Kodama Koi Farm lists possible causes of positive buoyancy disorder including gastrointestinal gas, parasites, a physical shift in the swim bladder, or tumors displacing the organ, and states that treatment may include surgery, syringe-based air removal, or other measures determined by a veterinary diagnosis.[1]In fish generally, the swim bladder (gas bladder) can be injured by pressure changes or acoustic trauma, with ruptured gas bladders and internal hemorrhage documented as resulting injuries in research on sound exposure in fish.[7] For more detail on buoyancy disorders in koi specifically, see the article on swim bladder disorders.

Pain and welfare considerations

Whether fish experience pain from injury has been studied experimentally. One widely cited study found that squid injured prior to encountering a predatory fish exhibited earlier defensive behavior than uninjured squid, an effect blocked by prior anesthesia, which researchers interpreted as evidence that nociceptive sensitization following injury is an adaptive response.[8] This line of research is cited in broader discussions of whether fish can feel pain, relevant to how injuries and handling of koi are approached.[8]

Diagnosis and when to seek professional help

Kodama Koi Farm recommends observing koi behavior, feces, water quality, and physical body condition — including skin texture, eyes, abdomen and gills — as part of a five-area check to narrow down the cause of a health problem.[1] It states that a basic microscope can help examine scales and parasites and is recommended as an affordable diagnostic tool for koi keepers.[1]UF/IFAS states that for conditions such as lymphocystis-like nodules, a wet-mount biopsy examined under a microscope can support a tentative diagnosis, but recommends working with a fish health professional or diagnostic laboratory for confirmation, since similar lesions can arise from parasites, bacterial disease, fungus, or cancers.[2]Kodama Koi Farm notes that a koi veterinarian can recommend diagnostic testing such as laboratory analysis or microscopic examination, helps identify the underlying cause of health issues rather than only addressing visible symptoms, and evaluates physical injuries from accidents, predators, handling or other fish.[6] Further background on diagnostic approaches is available in the article on diagnosis.

Common questions

Can koi get lymphocystis?
Sources disagree: a hobbyist-oriented symptom table from Kodama Koi Farm lists cauliflower-like growths on koi attributed to lymphocystis,[1] but University of Florida IFAS Extension states that true lymphocystis disease, caused by Lymphocystivirus, does not affect cyprinids including koi.[2]
Can koi get sunburned?
Sunburn is a documented radiation-burn condition from UV overexposure in animals generally,[3] and Kodama Koi Farm, a koi dealer, advises that shade from pond plants helps reduce the risk of koi sunburn and stress.[4]
What causes buoyancy problems and swollen abdomens in koi?
Kodama Koi Farm lists possible causes of positive buoyancy disorder including gastrointestinal gas, parasites, physical shifts in the swim bladder, and tumors, recommending veterinary diagnosis and possible treatments such as surgery or syringe-based air removal.[1]
When should an injured or growth-affected koi be seen by a veterinarian?
According to Kodama Koi Farm, a koi veterinarian can run diagnostic testing, identify underlying causes rather than just visible symptoms, and evaluate wounds or injuries from accidents, predators, handling, or other fish.[6]

References

  1. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  2. ^ UF/IFAS EDIS FA181: Lymphocystis Disease in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA181. Retrieved November 2026.
  3. ^ Sunburn (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Sunburn. Retrieved November 2026.
  4. ^ Best Types of Plants for a Koi Pond. Kodama Koi Farm (koi dealer). kodamakoifarm.com/plants-for-koi-ponds/. Retrieved November 2026.
  5. ^ Ultraviolet germicidal irradiation (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation. Retrieved November 2026.
  6. ^ When Should You Call a Koi Veterinarian? A Koi Owner's Guide to Professional Fish Care. Kodama Koi Farm (koi dealer). kodamakoifarm.com/fish-veterinarian/. Retrieved November 2026.
  7. ^ Swim bladder (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Swim_bladder. Retrieved November 2026.
  8. ^ Pain in fish (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Pain_in_fish. Retrieved November 2026.