Koi Fish 101: Varieties, Sizing & Stocking
Popular koi varieties, growth rates, stocking density rules, where to buy koi in SoCal, and how to introduce new fish safely.
Read Guide →Before You Buy
Choosing a healthy fish
Start with Signs of a Healthy Koi in our Koi Fish 101 guide. In short, sick fish often stop feeding and look lethargic, and fish that hang listlessly, gasp at the surface, or rub against objects are signalling a problem[6]. Sores, ragged fins, a swollen belly, or bulging eyes are physical signs that call for a closer look[6].
Questions to ask the seller
UF/IFAS suggests asking whether there have been any major unexplained losses in the population, and whether any KHV (koi herpesvirus) testing has been done — and if so, for a copy of the lab results[1]. The Merck Veterinary Manual also advises getting a history of previous treatments and disease outbreaks[3]. UF/IFAS adds that the best prevention is knowing your suppliers and having a good working relationship with them[1].
Why Quarantine Matters
Koi herpesvirus (KHV). KHV is highly contagious and often kills 80–100% of susceptible fish when water is between 60°F and 77°F (16–25°C)[1]. There is no treatment[1]. Koi that survive an outbreak are considered carriers, and carriers may show no signs while still carrying or shedding the virus[1]. Goldfish can also become infected and spread KHV to koi without showing signs themselves[1]. The virus spreads through direct contact with infected fish, their fluids, and contaminated water, mud, or equipment[1].
Parasites and bacteria. A newly moved koi is stressed and more susceptible to disease or parasite attack[10]. Merck notes that quarantine is most useful for catching external parasites and some internal parasites[3].
Limits. Some pathogens, such as mycobacteria and many viruses, can't be detected with current non-lethal tests, so quarantine lowers risk rather than eliminating it[3].
Setting Up a Quarantine Tank
Keep new fish in a separate system, ideally in a different building or area from your pond fish[1]. One koi club health advisor suggests about 50 gallons for one or two small koi, up to 1,000 gallons or more for several large ones[10].
Filtration. Provide both mechanical and biological filtration[10]. A biofilter takes several weeks to establish[10], so Merck suggests running a disinfected sponge filter in an established, disease-free system before you buy, to colonise it with nitrifying bacteria[3].
Aeration, heat and cover. Merck's basic quarantine kit (described for aquarium fish) is a tank, filter, aeration pump, and heater[3]. Cover the tank with a net; stressed koi can jump.
Water. Chloramine in tap water should be removed before the water goes into an aquarium or pond[12].
Biosecurity. Give the quarantine tank its own nets, buckets, and siphon hoses[1][3]. Care for your pond fish first, then the quarantined fish[1], and use foot baths and hand washes at the quarantine area[1]. Afterwards, break the system down, disinfect it, and store it dry[3][5].
| Quarantine setup checklist | Why | Source |
|---|---|---|
| Separate tank, away from the pond | Keeps water, splashes and equipment apart | [1] |
| Mechanical + biological filter, seeded ahead of time | Holds ammonia and nitrite down from day one | [3][10] |
| Air pump / aeration | Oxygen for stressed fish | [3] |
| Heater (and thermometer) | Holds about 75°F (24°C) for the KHV protocol | [1][3] |
| Net cover | Stressed koi can jump | — |
| Dechlorinator rated for chloramine | Chloramine should come out before water goes in | [12] |
| Dedicated nets, buckets, siphon | Equipment can carry pathogens | [1][3] |
| Ammonia and nitrite test kit | Catch water problems before they look like disease | [6][10] |
How Long, and at What Temperature
UF/IFAS recommends at least 30 days at about 75°F for KHV[1]; a veterinarian-written care sheet suggests four to six weeks[2]. Merck gives the same 30-day, 75°F minimum for koi[3][4], and its pet-owner guidance suggests 30 to 60 days for valuable fish[5].
Temperature matters because KHV's incubation period runs 7–21 days depending on water temperature[1]. In cool water (around 55°F/13°C) the virus can infect fish without causing signs, which can appear later once the water warms[1] — so a cold quarantine tank can hide the very thing you're looking for. Warmer isn't always better: UF/IFAS cautions that holding koi above 80°F may increase other bacterial and parasitic diseases[1].
Adding more fish partway through restarts the clock[5].
What to Watch For During Quarantine
General warning signs include going off food, lethargy, hanging listlessly, gasping at the surface, rubbing against objects, ulcers or bleeding spots, ragged fins, dropsy (swollen body), and popeye[6].
KHV signs are often non-specific, but the most common is damaged gills with mottled red and white patches[1]. Other signs can include bleeding gills, sunken eyes, pale skin patches, lethargic swimming near the surface, and laboured breathing[1]. Deaths can begin 24 to 48 hours after signs first appear[1], and secondary bacterial or parasite infections can mask the underlying virus[1].
If fish look unwell, test the water first[6]. Keep ammonia and nitrite near zero and feed sparingly; one koi club advisor calls food "the source of most quarantine problems"[10]. Any fish that becomes sick should be examined by a veterinarian or diagnostic lab to rule out KHV[1][3]; the American Association of Fish Veterinarians' "Find A Fish Vet" directory lists aquatic vets[1].
Testing: What It Can and Can't Tell You
Confirming KHV requires a veterinarian or fish health specialist working with a diagnostic laboratory[1]. PCR testing detects KHV DNA and can confirm the disease in sick fish, and perhaps identify carriers[1]. Non-lethal samples (blood, mucus, gill clips) can be tested, but those results may be less definitive or less accurate[1]. A negative result does not prove a fish is not a carrier — UF/IFAS states that no test definitively detects all carriers[1].
KHV is on the World Organisation for Animal Health (WOAH) disease list[1]. 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[13] (unlike spring viremia of carp, which USDA treats as a foreign animal disease in commercially produced fish[14]).
Bringing Koi Home: Transport and Bags
Shipped fish usually travel in sealed bags of water and pure oxygen[8]. Keep bags out of the sun: sunlight can quickly warm transport water, and high temperatures directly stress cool-water fish such as koi[7].
Inside a sealed bag, carbon dioxide lowers the pH, which keeps more of the fish's ammonia in its less toxic form[7]. As pH and temperature rise, more of it shifts to the toxic form[7]. So for fish bagged many hours, opening the bag and lingering can be risky.
Acclimation, Step by Step
You'll acclimate twice: once from the bag into quarantine, and again when moving fish from quarantine to the pond.
- Equalise temperature with the bag sealed. Float the unopened bag in shade[8]. Allow 20–30 minutes, with a damp towel over the bag in sun; University of Missouri Extension says at least 30 minutes[8]. Confirm with a thermometer. A Southern Regional Aquaculture Center guide advises at least 20 minutes per 10°F of difference, and warns that a sudden change of more than 10°F can harm fish[9].
- Mix waters gradually. University of Missouri Extension describes opening the bags, adding receiving water, and checking both temperature and pH while slowly adding more, to equalise them before release[8]. Mixing receiving water into the transport water is advisable when pH differs by 1 unit or more, or temperature by more than 10°F[9]. UF/IFAS describes adding it gradually, with an airline in the container, over two hours or more if temperatures differ by more than 10°C (18°F)[7]. For overnight-shipped fish, follow the shipper's instructions because of the ammonia issue above[7]; another approach is to float the bag, then net the koi out without mixing.
- Net the fish across — don't pour the bag water in. Net the koi out and keep bag water out of the tank[11]. KHV can travel in water[1], so discard it.
- Cover the tank and observe. Put the net cover on right away, then watch closely using the signs listed above.
| Timeline | What happens | Source |
|---|---|---|
| Day 0 | Float sealed bag in shade; equalise temperature; net fish into quarantine | [8][9] |
| Days 1–30+ | Hold at about 75°F (24°C); test water; observe daily; vet/lab for any sick fish | [1][6] |
| Weeks 4–6 | Preferred minimum end of quarantine | [2] |
| After quarantine | Optional "test" pairing with a few pond fish, at least 2 weeks | [1][4] |
| Move to pond | Acclimate again to pond temperature | [9] |
Moving Koi Into the Main Pond
UF/IFAS describes an optional final check: house several new koi with several pond koi in a separate area and watch for disease[1]; Merck suggests at least 2 weeks[4]. UF/IFAS is candid that "there are no guarantees"[1], and this deliberately exposes some pond fish to the newcomers. One koi club advisor instead puts a small pond koi into quarantine from the start[10] — that fish shares the newcomers' risk.
Your quarantine tank may be warmer or cooler than the pond, so check both temperatures and allow for the difference[9].
Southern California Note
Some local water suppliers use chloramine; Crescenta Valley Water District, for example, says it should be removed before water goes into a pond[12]. Ask your supplier and use a product rated for chloramine.
Sun is the other local factor: transport water warms quickly in sunlight, and heat stresses koi[7], so go straight home and float bags in shade[8].
KHV is temperature-dependent. UF/IFAS reports outbreaks typically at 60–77°F[1]; Merck gives 72–81°F for clinical disease[4]. Watch your pond thermometer — those ranges are when a hidden carrier is most likely to cause trouble.
FAQ
The seller says the fish were already quarantined. Do I still need to?
Yes. A negative test doesn't prove a fish isn't a carrier[1], and UF/IFAS calls quarantine the most dependable protection[1].
Do goldfish need quarantine too?
Yes. Goldfish can carry KHV and spread it to koi without looking sick[1][4].
Can I add another new fish halfway through?
Not without restarting the quarantine clock for the whole group[5].
My koi went to a show. Do they need quarantine when they come back?
Merck recommends following quarantine protocols every time koi return from a show[4].
Should I turn up the heat to "flush out" KHV?
One koi club article suggests raising quarantine water to 80°F for a few days[10]. We found no extension or veterinary source recommending that. UF/IFAS recommends 75°F for KHV quarantine and cautions that holding koi above 80°F can increase other diseases[1].
Sources
- UF/IFAS Extension, VM-149 "Koi Herpesvirus Disease (KHVD)", Petty, Hartman, Yanong, Pouder, Francis-Floyd, Waltzek
- PetMD, "Koi Fish Care Sheet", written by Melissa Witherell, DVM
- Merck Veterinary Manual (professional), "Management of Aquarium Fish", Petty et al., UF
- Merck Veterinary Manual (professional), "Viral Diseases of Fish"
- Merck Veterinary Manual (pet-owner version), "Routine Health Care of Fish", Francis-Floyd & Petty
- UF/IFAS Extension, CIR921 "Introduction to Fish Health Management", Francis-Floyd, Stilwell, Yanong
- UF/IFAS Extension, FA-119 "On-Farm Transport of Ornamental Fish", Crosby, Hill, Martinez et al.
- University of Missouri Extension, MX392 "Transportation of Fish in Bags" (reviewed April 2022)
- Southern Regional Aquaculture Center, SRAC Publication No. 392 "Transportation of Warmwater Fish: Procedures and Loading Rates" (1990; hosted by NCRAC)
- Midwest Pond and Koi Society, "Quarantining Your New Koi", Bryan Bateman, AKCA Certified Koi Health Advisor
- CARES Preservation Program, "Receiving Fish Step-by-Step"
- Crescenta Valley Water District (Los Angeles County), "Chloramine Info"
- 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)