Koi Pond Water Chemistry: pH, Ammonia & Nitrates Explained
What every test kit number means, safe ranges for SoCal koi ponds, and exactly how to fix each parameter when it goes wrong.
Read Guide →Step one: test the water
Poor water quality is a common and important stressor of cultured fish, and it precedes many disease outbreaks[1].
You can't see most water problems by looking. Raised ammonia or nitrite, swings in pH, and low oxygen are all invisible to the human eye[3]. Ammonia is colourless and has no smell, even at levels toxic to fish, so testing is the only way to know it is there[4]. Nitrite can be toxic at levels as low as 0.10 mg/L[4]. UF/IFAS recommends keeping dissolved oxygen at 5 mg/L or higher[1].
If fish start acting oddly, test before you change any water. Diagnostic labs want water sampled as soon as fish start acting abnormally and before water changes begin[3]. For target ranges and how to read a test kit, see our Koi Pond Water Chemistry guide. Keep a log, too: a fish health specialist will ask about your water tests, any new fish, your feeding, and treatments you have already tried[3].
Why diagnosis needs a microscope
Many koi problems look alike from the side of the pond. Other diseases can look like white spots, so white spots should never be the only basis for a diagnosis[6]. For many skin and gill parasites, a microscope exam of infected tissue is needed to confirm the diagnosis[2]. The usual method is a gentle skin scrape, sometimes with a small gill or fin clip, examined under a compound microscope[6]. Bacterial infections need laboratory culture and sensitivity testing to identify the bacteria and find an antibiotic that works[12]. Confirming KHV needs a diagnostic laboratory[18].
Quick reference: symptom, possible causes, first step
| What you see | Possible causes | Sensible first step |
|---|---|---|
| Rubbing or "flashing", extra slime | Flukes, costia, early Ich[6][7][8] | Test water; ask a vet for a skin scrape[2] |
| Small white spots like salt grains | Ich, though other diseases look similar[6] | Confirm under a microscope; Ich needs prompt treatment[6] |
| Hanging at the surface, fast breathing | Low oxygen, high ammonia, heavy gill parasites, KHV[1][4][8][19] | Test oxygen and ammonia; call a vet if fish keep struggling[3] |
| Red sores, ulcers, ragged fins | Bacteria such as Aeromonas, often after stress[2][12] | Test water; culture and sensitivity testing before antibiotics[12] |
| Grey-white cotton tufts | Water mold (Saprolegnia)[2] | Look for the underlying problem; check sanitation[2][17] |
| Swollen belly, pinecone-like scales | Dropsy: bacterial infection, SVC, other causes[2][21] | Call a vet[3] |
| Thread on the skin, or a flat clear disc | Anchor worm or fish lice[2][9][10] | Have a fish health professional confirm it[9][10] |
| Mottled red-and-white gills, sunken eyes, deaths | Possible KHV[18] | Call a fish vet now[18] |
External parasites
Ich (white spot)
Ichthyophthirius multifiliis is a single-celled parasite that causes "Ich" or white spot disease, and all freshwater fish are considered susceptible[6]. Before spots appear, fish may flash, produce extra mucus, lose appetite, and slow down[6]. By the time white spots are visible to the naked eye, the fish is very sick[6]. UF/IFAS calls an Ich outbreak a true emergency[6].
Only the free-swimming stage of Ich can be killed by treatment, so one dose won't do it; you need repeated treatments timed correctly[6]. Some treatments are risky: UF/IFAS describes copper sulfate as extremely toxic to fish, especially in low-alkalinity water[6]. Work with a vet rather than guess at doses.
Costia (Ichthyobodo)
Ichthyobodo, formerly called Costia, is a common parasite of the skin and gills of pond and aquarium fish[2]. Infected fish often produce excess mucus[7]. You need a microscope to see it, and under one it looks like a flickering flame[2][7].
Flukes (monogeneans)
What keepers call "flukes" are monogeneans, flatworms that feed on the skin, fins, and gills, and strictly speaking they are not flukes at all[8]. The gill-dwelling kinds are found mainly on cyprinids such as koi[8]. Heavy gill infestations cause swollen, pale gills, faster breathing, and less tolerance of low oxygen[8].
Anchor worm (Lernaea)
Anchor worms are not worms but crustaceans[2]. The adult female looks like a thin thread about 25 mm long, embedded in the skin[9]. Infestations are most common in summer, and most common in koi, carp, and goldfish[9]. The attachment site can become red and ulcerated, and secondary bacterial and fungal infections can follow[9]. Pulling them out with forceps doesn't always remove the anchor, so the whole system needs treatment[9].
Fish lice (Argulus)
Fish lice are flat crustaceans, especially common on goldfish, koi, and other carp[10]. Adults are 3–7 mm long, and infestations tend to peak in summer and autumn[10]. They can carry spring viraemia of carp (SVC)[10]. There are no FDA-approved drugs for fish lice, and raising the salt level does not appear to control them in fresh water[10].
Bacterial problems: ulcers and fin rot
Aeromonas is the most common bacterial infection of freshwater aquarium fish. Signs include bloody spots or ulcers, ragged fins, enlarged eyes, and fluid build-up in the belly[2]. Koi and goldfish are especially susceptible to Aeromonas salmonicida, which can cause deep ulcers and death[2]. Aeromonas and Pseudomonas are commonly isolated from freshwater fish with this kind of infection[11].
Bacterial outbreaks are frequently linked to poor water quality, handling and transport, sharp temperature changes, and low oxygen[11]. According to UF/IFAS, antibiotics alone do not cure a fish; poor water quality and other stresses have to be removed or reduced first, because bacterial infections are often secondary to them[12].
Antibiotics: the US rules
Under FDA Guidance for Industry #263, every new affected product entering the market since June 11, 2023 must carry prescription labelling[13]. The FDA has warned companies that illegally sold antibiotics such as amoxicillin, penicillin, tetracycline, and erythromycin over the counter for aquarium fish. It notes that all approved animal drugs containing these antimicrobials are available legally only by prescription from a licensed veterinarian[14]. The FDA also says that the antibiotics sold for ornamental fish in pet stores or online have not been approved, and that marketing them is illegal[15].
In practice, talk to a vet, who can run culture and sensitivity tests[12]. Antibiotics added to the water can also harm your biological filter; studies in recirculating systems found biofilter function reduced by as much as 44–100%[16].
Fungus-like infections (water mold)
Saprolegnia shows up as grey-white, cotton-like growths on the skin, gills, eyes, or fins[2]. It is often called a fungus, but it is a water mold (an oomycete)[10][17]. Researchers describe it as a widespread opportunistic pathogen. Stress, injury, and other infections make fish more susceptible[17]. Merck advises correcting poor sanitation and removing dead and decaying matter[2]. If you see cotton tufts, ask what damaged the skin first.
Dropsy
Dropsy is a build-up of fluid in the body cavity[18], seen as a swollen belly and sometimes "pinecone" scales[2]. It is a sign with several possible causes, including Aeromonas-type bacterial infections[2] and spring viraemia of carp[21]. Because the cause varies, get a vet's diagnosis.
Serious viral diseases: KHV and SVC
Koi herpesvirus (KHV)
KHV is a highly contagious virus of koi and common carp[18]. In fish never exposed to it before, it often kills 80–100% of the population when water is between 60°F and 77°F[18]. Signs include mottled red-and-white gills, sunken eyes, pale skin patches, and sometimes a notched nose[18]. Deaths can begin 24 to 48 hours after signs appear[18]. There is no treatment, and survivors should be considered carriers[18].
KHV is a WOAH-listed disease[18][20]. USDA's guidance for accredited veterinarians says cases must be reported to the area veterinarian in charge, although there is no federal response program for it[23] (unlike spring viremia of carp, which USDA treats as a foreign animal disease in commercially produced fish[24]).
Spring viraemia of carp (SVC)
SVC is a viral disease that affects koi, common carp, and goldfish[21]. Signs include lethargy, darkened skin, pop-eye, a swollen belly, pale gills, and bleeding in the skin, gills, and eyes[21]. Deaths generally occur between 11°C and 17°C (about 52–63°F)[21]. USDA treats SVC as a foreign animal disease in commercially produced fish in the United States[21][24], and diagnoses are reportable to USDA[21]. It is also known to occur in wild fish[24]. There is no antiviral treatment and no vaccine[21].
Prevention: quarantine
UF/IFAS recommends quarantining new fish for at least 30 days[6], and for KHV, keeping new koi in 75°F water for at least 30 days[18]. USDA training material suggests 4–6 weeks for hobbyists[19]. Give quarantined fish their own nets and buckets, and ask suppliers about unexplained losses and KHV testing[18].
Southern California note
Warm water speeds things up. At 75–79°F, Ich completes its life cycle in about 3 to 6 days[6]. Higher temperature and pH both increase the toxic share of ammonia[4]. Warm water also holds less dissolved oxygen than cold water[5]. Anchor worms do best at 26–28°C (about 79–82°F)[9]. If your pond runs warm, test more often and watch the fish closely. KHV outbreaks occur mainly between about 60°F and 77°F[18], so watch your thermometer during that range.
When and how to find a fish vet
Contact a vet or fish health professional as soon as you suspect a disease problem[3]. Don't wait if you see:
- signs of Ich, which can cause massive losses quickly[6]
- mottled gills, sunken eyes, or sudden deaths that could be KHV[18]
- ulcers, a swollen belly, or raised scales, which need lab testing[2]
Start with the American Association of Fish Veterinarians' "Find a Fish Vet" map[22]. If no fish vet is listed near you, AAFV suggests asking your regular vet to consult with a fish vet, contacting AAFV, or asking a local aquarium society for a recommendation[22]. Labs generally want fish that are sick or close to death, because dead fish are rarely acceptable for testing[3].
FAQ
My koi are flashing but otherwise look fine. Should I treat?
Test the water first, then get a skin scrape. Flashing can be a sign of flukes, fish lice, or early Ich[6][8][10], and only a microscope tells you which[2].
Can I just add salt?
Salt is one option for some problems, such as Ich in smaller systems[6], but it is not a cure-all. It does not appear to control fish lice in fresh water[10], and pond plants may not tolerate it[6]. Ask a vet first.
Can I buy fish antibiotics at a pet store?
The FDA says antibiotics sold for ornamental fish in pet stores or online are not approved and are marketed illegally[15]. Approved versions require a veterinarian's prescription[14].
Is KHV dangerous to people?
No. UF/IFAS states there is no zoonotic concern with KHV[18].
Can a koi that survived KHV rejoin my other fish?
Survivors should be treated as carriers that can infect other koi[18]. USDA leaves the decision to you and your vet[19].
Sources
- UF/IFAS EDIS, "Stress—Its Role in Fish Disease" (Circ. 919)
- Merck Veterinary Manual (Pet Owner version), "Disorders and Diseases of Fish," last updated Apr 2026
- UF/IFAS EDIS, "Submission of Fish for Diagnostic Evaluation" (FA-55)
- UF/IFAS EDIS, "Ammonia in Aquatic Systems" (FA-16)
- U.S. Geological Survey Water Science School, "Dissolved Oxygen and Water"
- UF/IFAS EDIS, "Ichthyophthirius multifiliis (White Spot) Infections in Fish" (Circ. 920)
- UF/IFAS EDIS, "Common Freshwater Fish Parasites Pictorial Guide: Flagellates" (FA-109)
- UF/IFAS EDIS, "Monogenean Parasites of Fish" (FA-28)
- UF/IFAS EDIS, "Lernaea (Anchorworm) Infestations in Fish" (FA185)
- UF/IFAS EDIS, "Argulus (Fish Louse) Infections in Fish" (FA184)
- Merck Veterinary Manual (Professional version), "Bacterial Diseases of Fish," R. Yanong, last updated Sept 2024
- UF/IFAS EDIS, "Use of Antibiotics in Ornamental Fish Aquaculture" (Circ. 84)
- U.S. FDA, "GFI #263: Frequently Asked Questions for Farmers and Ranchers" (content current as of 05/22/2024)
- U.S. FDA press release, Dec 14, 2023
- U.S. FDA, "Ornamental Fish Drugs and You"
- UF/IFAS EDIS, "Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 2: Pathogens" (Circ. 121)
- "Tracing the oomycete pathogen Saprolegnia parasitica in aquaculture and the environment," peer-reviewed (Scientific Reports, 2022, via PubMed Central)
- UF/IFAS EDIS, "Koi Herpesvirus Disease (KHVD)" (VM-149)
- the module splits its text across pages) (also aqdz0130.htm, aqdz0150.htm, aqdz0160.htm)
- World Organisation for Animal Health (WOAH), disease page
- UF/IFAS EDIS, "Spring Viremia of Carp Virus (SVCv)" (VM-142)
- American Association of Fish Veterinarians (AAFV), "Find a Fish Vet"
- USDA APHIS, National Veterinary Accreditation Program, Module 28 "Significant and Emerging Viral Diseases of Carp, Koi, and Goldfish" (KHV, response page)
- USDA APHIS, National Veterinary Accreditation Program, Module 28 "Significant and Emerging Viral Diseases of Carp, Koi, and Goldfish" (SVC, reporting page)