Signs of illness
Key facts: Signs of illness in koi (錦鯉) include behavioral changes such as flashing, jumping, gasping at the surface, clamped fins, and lethargy, as well as physical signs such as sores, ulcers, ragged fins, and abnormal body shape.[1][2]
Fish health management emphasizes prevention of disease over treatment, and daily observation of appearance, behavior, and feeding activity allows keepers to detect problems early, before a disease outbreak affects most of a population.[1] According to the University of Florida IFAS Extension, the most obvious sign of sick fish is the presence of dead or dying animals, but careful observers can usually tell fish are sick before mortalities begin because sick fish often stop feeding and may appear lethargic.[1]According to the US koi dealer Kodama Koi Farm, keepers can learn to recognize abnormal behavior by observing koi both from a distance, before the fish notice them, and at feeding time, since even a sluggish koi may rush toward a feeder it associates with food.[5] Warning signs cited by the same source include isolating from other fish, refusing food, gasping at the surface, flashing or rubbing against objects, erratic swimming, lethargy, constant hiding, and difficulty maintaining balance, and it notes that behavioral changes often appear before visible symptoms such as sores or discoloration.[2]Behavioral signs
The University of Florida IFAS Extension states that fish observed hanging listlessly in shallow water, gasping at the surface, or rubbing against objects indicate something may be wrong, since these behaviors show the fish is not feeling well or that something is irritating it.[1]Southern Regional Aquaculture Center guidance on investigating fish die-offs lists lying listlessly in shallow water or at the surface, inability to maintain equilibrium (turning on its side or upside down), and gulping water or air at the surface despite sufficient dissolved oxygen as abnormal behaviors worth noting when collecting a diagnostic sample.[3]According to Kodama Koi Farm, warning signs include isolating from other fish, refusing food, gasping at the surface, flashing or rubbing against objects, erratic swimming, lethargy, constant hiding, and difficulty maintaining balance, and prolonged bottom-sitting can indicate stress, poor water quality, illness, or low oxygen levels, especially when combined with clamped fins or reduced appetite.[2]Why koi jump, flash and gasp
According to Kodama Koi Farm, koi may jump out of the pond because of parasites, poor water quality, stress, sudden disturbances, or a lack of oxygen; occasional jumping can occur naturally, but repeated jumping may indicate something is irritating or stressing the fish, and keepers are advised to check water quality, aeration, and pond conditions if it continues.[2]Flashing, or rubbing against the pond bottom or sides, can indicate skin irritation caused by parasites, poor water quality, or other environmental problems; the same source notes that occasional flashing may not be serious, but frequent flashing should prompt a check of water parameters.[2] Kodama Koi Farm also states that several koi swimming erratically, rubbing along the wall, or jumping out of the water is often caused by parasites.[5]Gasping at the surface is associated with low dissolved oxygen or gill disease; University of Florida IFAS Extension identifies low oxygen as a frequent cause of fish mortality in ponds, especially in summer when water is warmer.[1] Kodama Koi Farm adds that hanging near a waterfall or area of water movement may reflect koi seeking higher oxygen levels due to distress.[5] A separate Kodama Koi Farm article on gill health states that koi with gill infections may show rapid gill movement, gasping at the surface, clamped fins, lethargy, or loss of appetite, with gills that appear swollen, pale, or covered in excess mucus.[6]Physical signs
In addition to behavioral changes, University of Florida IFAS Extension notes physical signs that should alert keepers to potential disease, including sores (ulcers or hemorrhages), ragged fins, or abnormal body conformation such as a distended abdomen ("dropsy") or protruding eyes ("exophthalmia" or "popeye").[1]The Southern Regional Aquaculture Center lists additional physical signs useful for diagnosis, including visible external or internal parasites, open sores, hemorrhaging, pustules, internal fluid accumulation (ascites) causing a swollen abdomen, bulging eyes, small red and white ulcers and pinpoint hemorrhages, eroded or bloody fins, excessive mucus on the body or gills, and swollen, fused, clubbed, or mottled gills.[3]Underlying causes: stress and water quality
University of Florida IFAS Extension describes stress as a condition in which chemical, biological, physical, or procedural stressors push a fish beyond its normal tolerance; prolonged stress severely limits the effectiveness of the immune system, increasing the opportunity for pathogens already present in the water to cause disease.[4] Stress disrupts protective barriers such as the mucus coat, scales, and skin, and can suppress the inflammatory response and antibody production.[4]Disease is rarely a simple association between a pathogen and a host fish; other factors, generally grouped under the term "stress," are usually needed for active disease to develop.[1] A minimum water-quality screen during a suspected illness should check dissolved oxygen, ammonia, nitrite, and pH, since high ammonia and low oxygen are commonly associated with disease outbreaks, especially when fish are crowded.[1]Signs linked to specific conditions
- White spot (Ich): prior to visible white spots, fish may show irritation, flashing, increased mucus, weakness, loss of appetite, and decreased activity; by the time spots are visible, the fish is already very sick, and if the parasite is limited to the gills, fish may die in numbers with no visible spots.[7]Fish lice (Argulus): infested fish may show pinpoint hemorrhages, anemia, fin and scale loss, increased mucus, lethargy, erratic swimming, reduced feeding, hanging at the surface, and flashing to relieve irritation; in some cases there are no visible signs besides the parasite itself.[8]Columnaris: acute cases can show shortness of breath, gill discoloration, and flashing, while chronic cases show fin rot, saddleback-pattern lesions at the dorsal fin base, skin ulcerations, discolored gills, rapid labored breathing, anorexia, and lethargy.[9]Koi herpesvirus (KHV): the World Organisation for Animal Health describes focal or total loss of epidermis, patchy pale or reddened skin, abnormal mucus secretion, sunken eyes, hemorrhages, fin erosion, lethargy, separation from the shoal, gathering at the water inlet or pond sides, gasping at the surface, and in some fish loss of equilibrium or hyperactivity.[10]Gill-related illness: rapid gill movement, gasping, clamped fins, lethargy, and loss of appetite, with gills appearing swollen, pale, or covered in excess mucus, according to Kodama Koi Farm.[6]
Responding to suspected illness
University of Florida IFAS Extension recommends checking water quality first if fish are suspected to be getting sick, with a water quality test kit, and notes that a dissolved oxygen meter is considered essential for commercial operations.[1] Good records of behavioral and physical signs, together with water-quality data, provide a complete case history useful to a diagnostician.[1]The Southern Regional Aquaculture Center advises that, while awaiting diagnostic results, the best ways to support remaining fish are to improve water quality and reduce stocking density, including increasing aeration and filtration and adding small quantities of salt for freshwater animals.[3] It also recommends submitting several live, sick fish along with a water sample for disease or parasite diagnosis, since a single fish may not represent the whole population.[3]Viral diseases are impossible to distinguish from bacterial diseases without laboratory tests, and University of Florida IFAS Extension recommends consulting an aquaculture or fish health specialist if a bacterial or viral disease is suspected.[1] Readers can review related pages on quarantine practice for new arrivals and on diagnosis methods such as skin scrapes and microscope examination.[11]Common questions
What does it mean if a koi is flashing or rubbing against the pond?
Flashing, where a koi rubs or scrapes its body against the pond bottom or sides, can indicate skin irritation from parasites, poor water quality, or other environmental problems; occasional flashing may not be serious, but frequent flashing should prompt a water-quality check.[2] Several koi flashing and swimming erratically together is often linked to parasites.[5]
Why do koi jump out of the water?
According to Kodama Koi Farm, koi may jump because of parasites, poor water quality, stress, sudden disturbances, or a lack of oxygen; occasional jumping can be natural, but repeated jumping may signal irritation or stress requiring a check of water quality and aeration.[2]
Is gasping at the surface always a sign of low oxygen?
Gasping at the surface is associated with low dissolved oxygen, which the University of Florida IFAS Extension identifies as a frequent cause of mortality especially in warm water, but it can also reflect gill disease or gill infection, as noted by Kodama Koi Farm, so water quality and gill condition should both be checked.[1][6]
What are clamped fins a sign of?
Clamped fins, where the fins are held close to the body, are listed as a sign associated with stress, poor water quality, or parasitic infection, often appearing alongside lethargy, gasping, or loss of appetite.[5]
Should sick koi be treated before checking water quality?
University of Florida IFAS Extension recommends checking water quality first when fish are suspected of being sick, since medications cannot substitute for proper husbandry and many outbreaks are tied to water-quality problems such as low oxygen or high ammonia.[1]
Related koi topics
References
- ^ UF/IFAS EDIS FA004: Introduction to Fish Health Management. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA004. Retrieved November 2026.
- ^ Understanding Koi Behavior: Can Koi Recognize Their Owners, Hear, See Color, Sleep, and More. Kodama Koi Farm (koi dealer). kodamakoifarm.com/understanding-koi-behavior/. Retrieved November 2026.
- ^ 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.
- ^ UF/IFAS EDIS FA005: Stress—Its Role in Fish Disease. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA005. Retrieved November 2026.
- ^ 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.
- ^ Why Koi Gills Are Vital for Koi Health. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-gills-health/. Retrieved November 2026.
- ^ 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.
- ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.
- ^ Columnaris (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Columnaris. Retrieved November 2026.
- ^ Manual of Diagnostic Tests for Aquatic Animals, ch. 2.3.6 Koi herpesvirus disease (WOAH). World Organisation for Animal Health. woah.org/fileadmin/Home/eng/Health_standards/aahm/current/2.3.06_KH…. Retrieved November 2026.
- ^ How Long Do Koi Fish Live? What is Lifespan & Growth?. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-long-do-koi-fish-live/. Retrieved November 2026.