Skip to content

Gill problems

By the koi.wiki editorial team · Updated

Key facts: Gill problems in koi describe damage to the respiratory organ caused by parasites, bacteria, viruses, poor water quality or low dissolved oxygen, often producing gasping, lethargy and rapid gill movement[1].
The gills of koi perform gas exchange, osmoregulation and ammonia excretion, making them essential to survival beyond simple breathing[1]. Because gill tissue is hidden inside the gill cover, damage can progress unnoticed until a fish dies suddenly, with gill failure only discovered on post-mortem examination[1].Gill disease in koi and other cultured fish can arise from parasitic infestation, bacterial infection, viral disease, chemical or medication damage, or simple oxygen depletion in the water itself[1][7]. Several distinct and unrelated conditions can all present with similar outward signs, so accurate diagnosis generally requires microscopic examination of gill tissue[1].

Causes of gill damage

According to the US koi dealer Kodama Koi Farm, common causes of gill damage include poor water quality, parasites, bacterial infections, improper handling, and overuse or misuse of medications[1]. External parasites such as monogeneans (including the gill and skin flukes Dactylogyrus and Gyrodactylus) are among the most common organisms found attached to fish gills[2][3]. A table circulated by the same dealer lists gill flukes, bacterial gill disease, and gill hyperplasia as separate entries, with gill hyperplasia attributed to chronic irritation or poor water quality rather than a single pathogen[3].In farm-raised channel catfish, a parasitic condition called proliferative gill disease (also known as hamburger gill disease) causes severe gill swelling and broken gill cartilage, and is described by the Southern Regional Aquaculture Center as the leading parasitic disease of that industry, caused by a myxosporean parasite with an intermediate oligochaete worm host[8]. While this disease is documented primarily in catfish, the same aquatic-worm intermediate hosts found in pond bottoms are noted elsewhere as carriers of myxozoan parasites relevant to pond biosecurity generally[9].

Viral and bacterial causes

Koi herpesvirus (KHV) frequently produces severe gill lesions, appearing as gill mottling with red and white patches caused by necrosis of gill tissue; this is described as the most common clinical sign of the disease according to University of Florida IFAS Extension[4]. The World Organisation for Animal Health notes that gills are among the organs where KHV is most abundant during clinical disease, along with the kidney, gut and spleen, and that microscopy reveals erosion and fusion of gill lamellae along with necrosis[10]. The Southern Regional Aquaculture Center describes KHV-affected gill patches as white, yellow or brown, and notes that affected fish swim lethargically and congregate near aerators to obtain more oxygen through damaged gills[11].Carp edema virus (CEV), associated with koi sleepy disease, also produces histologic gill lesions, including hyperplasia and clubbing of secondary gill lamellae with partial or total occlusion of the interlamellar space, according to a World Organisation for Animal Health disease card[5]. Swollen gills or gill necrosis are listed among the gross lesions of CEV infection, alongside lethargy, sunken eyes and skin lesions[5]. The same source notes that mortality from CEV can be significantly reduced by immersing infected fish in a 0.5% saline bath[5].Columnaris disease, caused by the bacterium Flavobacterium columnare, commonly produces brown to yellowish-brown lesions on the gills as the bacteria attach to the gill surface and spread across individual filaments, according to the Southern Regional Aquaculture Center[6]. Wikipedia notes that columnaris in young fish is acute and damages the gills, leading to death by respiratory distress, with signs including shortness of breath, gill discoloration and rapid, labored breathing[12].

Low dissolved oxygen and water quality

Because gills absorb oxygen directly from water into the bloodstream, low dissolved oxygen (DO) can cause respiratory distress even when gill tissue itself is not diseased, according to University of Florida IFAS Extension[7]. A concentration of 5 mg/L DO or more is recommended for optimum fish health, with most species showing distress at 2–4 mg/L and mortality typically below 2 mg/L[7]. Warm water holds less oxygen than cool water, and fish metabolic demand for oxygen rises with temperature, which is why oxygen depletion events are most common during hot, cloudy, still summer weather[7].Signs that a fish kill may be caused by low oxygen include fish gasping at the surface (“piping”), fish dying with gills flared and mouths open, and a sudden change in water color signifying an algal bloom crash[7]. The recommended emergency response is to turn on aeration immediately; monitoring dissolved oxygen with an electronic meter or test kit can help predict and prevent depletion events[7].

Signs and symptoms

According to Kodama Koi Farm, koi with gill infections may show rapid gill movement, gasping at the surface, clamped fins, lethargy or loss of appetite, and gills may appear swollen, pale or covered in excess mucus[1]. A dealer-compiled symptom table lists gasping, clamped fins, reduced appetite and excess mucus as signs of gill flukes specifically, and gill mottling as a sign associated with KHV[3]. Healthy gills are described as bright red and evenly shaped; discoloration, swelling or necrosis are considered abnormal and worth photographing for consultation with a koi health professional[1].A koi swimming abnormally, such as gasping, tilting or isolating itself, with no visible ulcers or body injuries, should raise suspicion of gill damage first, according to Kodama Koi Farm[1]. The source also cautions that if one koi dies from gill-related issues, others in the same pond may already be affected[1].

Diagnosis

Diagnosis of gill disease generally starts with checking water quality parameters—ammonia, nitrite, pH and temperature—followed by microscopic examination of gill or skin scrapings for parasites, according to Kodama Koi Farm[1]. Diagnosis of columnaris and KHV can be confused because the conditions can mimic one another, so careful differential diagnosis is needed, according to the Southern Regional Aquaculture Center[11]. For proliferative gill disease in catfish, low-magnification microscopy of the gills is used to reveal swelling, cartilage gaps (chondrolysis) and the characteristic “mashed” appearance[8]. See Diagnosis: scrapes, microscopes and fish vets for general approaches to koi disease workup, and consult a fish veterinarian for conclusive diagnosis where sources recommend it[1].

Treatment and prevention

Nutrition and chronic gill irritation

University of Florida IFAS Extension lists gill disease among several nutritional diseases of fish that are generally diagnosed by exclusion, after ruling out other explanations and critically reviewing the feeding program[15]. Separately, gill hyperplasia—thickened gill tissue with labored breathing—is attributed to chronic irritation or persistently poor water quality rather than a single infectious agent, according to a dealer-compiled disease list[3].

Common questions

What does it mean if a koi is gasping at the surface?
Gasping at the surface, sometimes called piping, can indicate low dissolved oxygen in the water or damaged gills that cannot extract enough oxygen, according to University of Florida IFAS Extension and Kodama Koi Farm[7][1].
Can gill problems kill a koi without other visible symptoms?
Yes; Kodama Koi Farm notes that koi can die suddenly from gill failure with no visible disease elsewhere on the body, which is why gills should be examined after an unexplained death[1].
Is gill mottling always a sign of koi herpesvirus (KHV)?
No. Gill mottling with red and white patches is a common sign of KHV, but the Southern Regional Aquaculture Center notes it can resemble columnaris disease, so laboratory diagnosis is needed to distinguish them[4][11].
Does salt help treat gill flukes in koi?
Some koi keepers report raising salt levels to treat gill flukes alongside medications like praziquantel, but reader comments on a dealer site show disagreement about the correct percentage and duration, and salt is generally recommended only during recovery rather than as a long-term measure[1][14].
What should I check first if my koi is breathing rapidly?
Kodama Koi Farm recommends checking water quality (ammonia, nitrite, pH, temperature) and examining gill or skin scrapings under a microscope for parasites before assuming a specific disease[1].

References

  1. ^ Why Koi Gills Are Vital for Koi Health. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-gills-health/. Retrieved November 2026.
  2. ^ Fish diseases and parasites (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fish_disease_and_parasites. Retrieved November 2026.
  3. ^ 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.
  4. ^ UF/IFAS EDIS VM113: Koi Herpesvirus Disease (KHVD). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM113. Retrieved November 2026.
  5. ^ Carp edema virus disease card (WOAH, 2022). World Organisation for Animal Health. woah.org/app/uploads/2023/03/a-woah-cev-disease-card-nov-2022.pdf. Retrieved November 2026.
  6. ^ SRAC Publication No. 479b: Columnaris Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/479b%20Columnaris%20Disease-%20…. Retrieved November 2026.
  7. ^ UF/IFAS EDIS FA002: Dissolved Oxygen for Fish Production. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA002. Retrieved November 2026.
  8. ^ SRAC Publication No. 475: Proliferative Gill Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/475%20Proliferative%20Gill%20Di…. Retrieved November 2026.
  9. ^ SRAC Publication No. 4712: Biosecurity in Aquaculture, Part 3: Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4712%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  10. ^ 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.
  11. ^ SRAC Publication No. 4710: Herpesviruses in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4710%20Herpesviruses%20in%20Fis…. Retrieved November 2026.
  12. ^ Columnaris (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Columnaris. Retrieved November 2026.
  13. ^ What is Koi Sleeping Disease? KSS, KSD, Carp Edema Virus (CEV) Symptoms & Treatment. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sleeping-disease/. Retrieved November 2026.
  14. ^ Benefits of Salt in Koi Pond & Tips for Safely Using It!. Kodama Koi Farm (koi dealer). kodamakoifarm.com/benefits-salt-koi-pond-why-should-use-it/. Retrieved November 2026.
  15. ^ UF/IFAS EDIS FA096: Fish Nutrition. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA096. Retrieved November 2026.