Skip to content

Diagnosis: scrapes, microscopes and fish vets

By the koi.wiki editorial team · Updated

Key facts: Diagnosis of koi disease relies on a combination of case history, water testing, and microscopic examination of skin, fin and gill biopsies, with laboratory culture or histology used when a problem cannot be identified by microscope alone[1][2].
Determining why a koi is sick is often described by fish health authorities as a process that goes beyond looking at a single symptom. The University of Florida IFAS Extension notes that without a thorough diagnostic workup, disease problems are often misdiagnosed, leading to inappropriate treatment that can cause more harm than good and delay an accurate diagnosis[1].A typical diagnostic approach combines a review of pond history and water quality records with direct examination of the fish, including skin scrapes, fin clips and gill clips viewed under a microscope[2][5]. When the cause remains unclear, samples may be sent to a fish disease diagnostic laboratory or evaluated by a fish veterinarian[1][3].

History, records and water testing

Diagnostic workups typically begin with case history: recent water changes, new fish introductions, feeding changes, and seasonal shifts[4]. According to the University of Florida IFAS Extension, a diagnostic laboratory will test water chemistry parameters such as total and unionized ammonia, nitrite, pH, total hardness, total alkalinity, and chloride as part of a general evaluation, with chlorine, dissolved oxygen, hydrogen sulfide, iron, and copper measured when warranted[1].According to the US koi dealer Kodama Koi Farm, before contacting a veterinarian it is useful to gather water test results (ammonia, nitrite, nitrate, pH, KH, GH, and temperature), pond volume, filtration details, photos or videos of the affected fish, the number of fish showing symptoms, and a timeline of when symptoms appeared[4]. Keeping good records of water chemistry, water changes and new additions is described as helping a fish health professional reach an accurate diagnosis[1].

Skin scrapes, gill clips and fin clips

External examination usually involves biopsies of skin, fin and gill tissue, examined under a light microscope primarily to look for parasites, though some bacteria, fungi and effects of water quality or nutrition can sometimes be assessed this way[1][2]. For suspected white spot disease (Ich, caused by Ichthyophthirius multifiliis), the University of Florida IFAS Extension describes lightly scraping a glass coverslip down the side of the fish from head to tail to remove skin cells and mucus, without removing scales, and separately clipping small pieces of fin and gill with sharp scissors[6]. Tissue samples are mounted in a drop of tank or other fresh (not distilled, deionized, reverse-osmosis, or chlorinated) water under a coverslip and examined at 20 to 400x magnification[6][7].Monogenean parasites, such as flukes, are similarly diagnosed by biopsies of fin, gill and skin mucus examined with a light microscope; because monogeneans do not survive long after a fish dies, examination of a live or freshly dead fish is recommended[8]. Anchor worm (Lernaea) can often be seen with the naked eye as a thread-like structure, but a fish health professional should confirm the diagnosis with a microscope, since the organism can be confused with plant fibers or fungi[9]. Lymphocystis is typically diagnosed by a fresh wet mount biopsy of wart-like growths examined under the microscope, revealing round to oval, grape-like cell structures[10]. Columnaris, caused by Flavobacterium columnare, is presumptively diagnosed by scraping an affected area, preparing a wet mount, and examining it at 100 to 400x magnification, with more definitive identification possible via antibody or PCR testing[11][12].

Identifying organisms under the microscope

Several protozoan parasites common in freshwater fish can be distinguished by size, shape and movement under magnification. Trichodina appears as a “flying saucer” shape roughly 35–135 micrometers in diameter and moves with a whirling motion, while Chilodonella is ovoid or kidney-shaped, dorsoventrally flattened, and roughly 30–80 by 20–60 micrometers[13]. Ichthyophthirius multifiliis trophonts range from about 50 micrometers to 1 millimeter and move with a rolling, amoeboid motion, with a distinctive C-shaped nucleus visible in adults[13][7]. Ichthyobodo (formerly Costia) is described as roughly the size of a red blood cell, shaped like a comma or teardrop, and can easily be missed under 100x magnification, especially with an average-quality microscope[14].Diagnosticians note that infections such as Trichodina cause no distinctive lesions, so diagnosis is made almost exclusively by microscopic examination, and the experience of the examiner in weighing overall parasite load alongside other environmental factors is important in determining whether a particular organism is actually causing disease[14].

Internal examination and necropsy

When circumstances allow, a full necropsy may be performed, sacrificing fish and examining external and internal tissues with and without a microscope, including microbiology to rule out bacterial, viral and fungal disease and to identify internal parasites[2]. At a diagnostic laboratory, fish are measured and weighed, abnormal external changes recorded, and internal organs examined for lesions; the kidney is sampled for bacterial culture, with other organs such as brain, liver, spleen or swim bladder cultured as needed[1]. Squash preparations of internal organs may also be examined under the microscope for parasites[1].Growth of bacteria on culture media usually takes 24 to 48 hours, with antibiotic sensitivity testing adding another 24 to 48 hours; tissues submitted for histology or virology commonly take one to two weeks for results[1]. Positive diagnosis of koi herpesvirus (KHV) requires the assistance of a veterinarian or fish health specialist and a fish disease diagnostic laboratory, using direct methods that detect virus or viral genetic material[15]. See also Koi herpesvirus (KHV).

Sample quality and timing

How a sample is collected and handled strongly affects how useful it is. Live moribund fish are considered the best specimens for identifying parasites, bacteria, fungus and viruses, and fish that will not survive transport can still be useful if iced immediately and examined within 24 hours[1]. Fish dead for more than about six hours are generally described as useless for most external parasite checks, bacterial isolation, or histology, since parasites fall off after death and decomposition allows bacteria to overgrow tissues[1].Gill color is one practical field indicator: if the gills are still bright red, a dead fish may be a valid sample if placed on ice immediately, but if gills are bright pink the sample quality has deteriorated substantially, and gills that are light pink, white or gray render the fish useless for diagnostic testing[3]. Fish that have floated to the surface of a pond are also considered useless for diagnostic purposes[3]. A minimum of three to six sick fish with obvious signs of disease, plus three to six fish showing abnormal behavior, are recommended for submission, since a single fish is not representative of a population and most outbreaks involve more than one problem[3]. Samples for disease or parasite diagnosis should never be frozen, as freezing renders them useless for pathology; instead they should be shipped on ice (not frozen) or, for toxicology testing, frozen fish may be appropriate[3].

When to involve a veterinarian or diagnostic laboratory

Authorities recommend contacting a veterinarian, fish health professional, or fish disease diagnostic laboratory as soon as a disease problem is suspected, rather than attempting treatment first, to avoid misdirecting a fish health management plan[1]. Establishing communication with a diagnostic laboratory before a die-off occurs, and posting its contact information in advance, is also recommended for pond or facility operators[3].According to Kodama Koi Farm, a US koi dealer, a fish veterinarian typically reviews pond history, water quality records, feeding practices and recent fish introductions, observes the fish’s behavior and appearance, and may recommend laboratory or microscopic testing when the cause of illness is not immediately apparent[4]. The source also states that not every sick koi requires medication, and that identifying the root cause through evaluation may be preferable to symptom-based treatment[4]. Hobbyists seeking a specialist can consult directories such as the American Association of Fish Veterinarians' Find A Fish Vet listing, as referenced by the University of Florida IFAS Extension[15]. See also quarantine and signs of illness for related early-warning context.

Diagnosing gill problems

Because gills are hidden from view, damage can go unnoticed until a koi is already in respiratory distress, according to Kodama Koi Farm[5]. The source describes rapid gill movement, gasping at the surface, clamped fins, lethargy and loss of appetite as signs of gill infection, with gills potentially appearing swollen, pale, or covered in excess mucus[5]. Recommended steps include checking water quality parameters (ammonia, nitrite, pH, temperature), examining gill or skin scrapings under a microscope for parasites, and ensuring adequate aeration[5]. Protozoan parasites such as Ichthyobodo can line up along the gill edge, giving it a serrated appearance under the microscope, and may be missed with a low-quality microscope[14]. See also gill problems.

Common questions

What is a skin scrape and why is it used?
A skin scrape involves lightly running a glass coverslip or scalpel down the fish's side to collect skin cells and mucus, which are then mounted on a microscope slide to check for external parasites such as Ichthyophthirius multifiliis or Trichodina[6][13].
Can a dead koi still be used for diagnosis?
It depends on freshness: fish dead more than about six hours are generally considered useless for parasite checks, bacterial isolation, or histology, and gill color (bright red versus pale pink or white) is used as a practical indicator of sample quality[1][3].
Should I treat a sick koi before getting a diagnosis?
Fish health authorities caution that starting treatment before an accurate diagnosis is made can compromise the fish's immune system, mask the original problem, and lead to secondary infections[1].
When should I call a fish veterinarian instead of treating at home?
According to Kodama Koi Farm, veterinary input is useful when problems persist, multiple fish are affected, or the cause of illness is unclear, since a veterinarian can recommend laboratory or microscopic testing beyond routine husbandry[4].
How long does laboratory diagnosis typically take?
Bacterial culture usually takes 24 to 48 hours, with antibiotic sensitivity testing adding another 24 to 48 hours, while histology or virology results commonly take one to two weeks[1].

References

  1. ^ UF/IFAS EDIS FA055: Submission of Fish for Diagnostic Evaluation. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA055. Retrieved November 2026.
  2. ^ UF/IFAS EDIS FA099: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 1. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA099. Retrieved November 2026.
  3. ^ SRAC Publication No. 472: Investigating a Fish Die-Off and Submitting a Sample. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/472%20Investigating%20a%20Fish%…. Retrieved November 2026.
  4. ^ 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.
  5. ^ Why Koi Gills Are Vital for Koi Health. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-gills-health/. Retrieved November 2026.
  6. ^ UF/IFAS EDIS FA006: Ichthyophthirius multifiliis (White Spot) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA006. Retrieved November 2026.
  7. ^ Ichthyophthirius multifiliis (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Ichthyophthirius_multifiliis. Retrieved November 2026.
  8. ^ UF/IFAS EDIS FA033: Monogenean Parasites of Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA033. Retrieved November 2026.
  9. ^ UF/IFAS EDIS FA185: Lernaea (Anchorworm) Infestations in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA185. Retrieved November 2026.
  10. ^ UF/IFAS EDIS FA181: Lymphocystis Disease in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA181. Retrieved November 2026.
  11. ^ SRAC Publication No. 479b: Columnaris Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/479b%20Columnaris%20Disease-%20…. Retrieved November 2026.
  12. ^ Columnaris (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Columnaris. Retrieved November 2026.
  13. ^ UF/IFAS EDIS FA108: Common Freshwater Fish Parasites Pictorial Guide: Motile Ciliates. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA108. Retrieved November 2026.
  14. ^ SRAC Publication No. 4701: Protozoan Parasites. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4701%20Protozoan%20Parasites.pdf. Retrieved November 2026.
  15. ^ UF/IFAS EDIS VM113: Koi Herpesvirus Disease (KHVD). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM113. Retrieved November 2026.