A koi quarantine is four weeks in a separate, filtered, aerated tank held at 74 to 78°F, with salt at 0.3%, daily ammonia and nitrite tests, and a fish you have watched eat normally for fourteen straight days before it ever sees your pond. Skip the heat and you have a holding tub. Skip the four weeks and you have a coin flip. This is the protocol we hand customers at the counter.
Short answer: Quarantine every new koi for 28 days in at least 150 gallons held at 74 to 78°F. Hold salt at 0.3% (2.5 lb per 100 US gallons), test ammonia and nitrite daily, and treat only what you can diagnose, with one exception: a praziquantel course for flukes. Release after 14 symptom-free, unmedicated days with zero ammonia and nitrite.
Why koi quarantine catches problems your pond cannot
A new koi arrives carrying whatever it picked up in its last few months: a low-grade fluke burden, a few Costia, a nick on the flank that has not yet turned into an ulcer. In a quarantine tub those surface within days, because the fish sits in shallow clear water and you look at it twice a day. In a 3,000-gallon pond, the first sign is one fish flashing on the bottom drain three weeks later. Medicating 150 gallons also costs a few dollars; medicating 3,000 gallons costs twenty times that and can flatten your filter.
ChampKoi koi are hand-selected in Japan and clear USDA APHIS import inspection, and the Koi Health Guarantee page sets out what that screening covers, including KHV and spring viremia of carp. It still tells you to quarantine at home. Screening lowers the odds of a virus; it cannot tell you how one fish will handle two days in a bag, and stress is what turns a carried pathogen into a sick fish.
From the shop: The most common quarantine failure we hear about is not a disease. It is a tub set up the day the fish landed, filled with uncycled tap water, with no test kit in the house. Build the tank first, buy the fish second.
Why four weeks, not two
Two weeks covers the fast problems. Most protozoan parasites reproduce in days, and a fish that arrived visibly sick declares itself in 72 hours. It does not cover the slow ones. Koi herpesvirus incubates for roughly 3 to 21 days at permissive temperatures, so a 14-day window can close the day before an outbreak starts. Fluke eggs laid before the fish shipped hatch after your first treatment. And a medicated fish needs an unmedicated window afterwards, or you release a koi whose symptoms are suppressed rather than resolved.
Twenty-eight days covers all three. UC Davis recommends at least six weeks at virus-permissive temperatures for a strict veterinary quarantine, so longer is better. Four weeks is the shortest we will defend, and one rule goes with it: adding another fish resets the clock to day one for everybody in the tank.
The temperature rule most quarantine advice leaves out
Quarantine only works if pathogens can express. Hold the tank at 74 to 78°F for the entire period.
Koi herpesvirus expresses between about 61 and 81°F, with peak mortality 72 to 78°F. Below roughly 61°F it goes quiet and infected fish show nothing. Run 28 days in a 55°F garage in March and a carrier walks through clean, into your pond, and breaks out in May once the water warms. That is the most expensive mistake in this article.
Warm water helps twice over: koi immune function is strongest above about 70°F, and parasite life cycles run fast at 76°F, so a light infestation shows up inside your observation window rather than after it. No single temperature catches everything, and spring viremia of carp does the reverse, showing at 50 to 63°F, and in fry as high as 72°F. We still take the warm band, because KHV is the disease that empties ponds.
Getting there means a thermostatic submersible heater with a guard. Budget about 5 watts per gallon for the lift you actually need: roughly 500 W for a 100-gallon tub and 750 W for a 150-gallon tub in an unheated garage, less indoors. Insulate the sides and keep a lid on it, because heat leaves through the surface faster than anywhere else. Read the water with a thermometer rather than the heater dial, and confirm it holds 74°F before the fish arrives. ChampKoi does not sell heaters, so buy that item from an aquarium supplier.
Sizing and equipping a koi quarantine tank
Use 150 gallons as the working minimum for one or two adult koi. You will see 100 gallons quoted, including in the older version of this guide, and 100 is fine for a pair of tosai under 10 inches. It is too tight for a 20-inch fish and for the ammonia that fish produces. Depth wants at least 2 feet, and the tank needs a weighted lid: a fish on the garage floor is a more common quarantine death than any disease.
Run air 24 hours a day, since warm salted water holds less oxygen than a cool pond, and control ammonia with a seeded biofilter or daily water changes. Any food-safe container works: a stock tank, a show vat, a cut-down tote. Nobody regrets the bigger tub.
Cycling the tank fast, and the media rule
Start the tank one to two weeks before the fish arrives. Fill it, condition the water, run heat and air, seed the filter with mature media from your main pond plus a culture such as Microbe-Lift PL, and feed it a small ammonia source until ammonia and nitrite clear to zero within 24 hours of a dose.
Never return that seeded media to your main pond. Not the sponge, not the mats, not the water, not the last bucketful when you drain the tub. It has spent a month soaking in whatever the new fish brought, and putting it back defeats the whole exercise. Bin it, or bleach it at one part bleach to nine parts water, rinse hard and dry it in sun. Media is cheap; buy a second set.
If you cannot cycle in time, run the tank with no biofilter, change 10 to 15% of the water daily and dose an ammonia-detoxifying conditioner such as Seachem Pond Prime each time. That works, but only if you do it every day.
Salt at 0.3%: the math and what it buys
Salt goes in every quarantine tank we run. The target is 0.3%, which is 3 grams per litre, or 2.5 lb of salt per 100 US gallons, so a 150-gallon tub takes 3.75 lb. The commonly quoted "3 lb per 100 gallons" lands nearer 0.35%, still safe, but measure your real volume rather than the tub's rim capacity. Add it in thirds over three days, dissolved in a bucket of tank water first. Salt does not evaporate and the filter does not remove it, so replace only what you take out: a 15% water change gets 15% of the dose back.
Koi blood sits near 0.9% salinity, so in fresh water a fish constantly pumps water back out. Raising salinity to 0.3% cuts that osmotic workload, supports the slime coat, kills several protozoans outright including Costia and Chilodonella, and reduces nitrite toxicity in a tank that is not fully cycled. It does nothing to flukes, nothing to KHV and nothing to an established ulcer, and it kills aquatic plants. Take it back out with water changes at the end rather than salting your pond to match.
Prophylactic dosing versus treating what you diagnose
Experienced keepers genuinely disagree here, and the older version of this article took both sides in consecutive bullets. Here is where we land. Two things go in prophylactically: salt at 0.3%, and one praziquantel course. Everything else waits for evidence.
Praziquantel earns its place because gill and skin flukes are close to universal on shipped koi, invisible without a microscope, and cause exactly the low-grade damage a secondary bacterial infection exploits. It is also gentle on fish and filter. Aqua Prazi doses at 1 gram per 100 gallons, so a 25 g pack treats 2,500 gallons, a little over 2 ppm. Repeat three days later to catch larvae hatching from eggs the first dose missed, and do not water-change for seven days from the first treatment. Switch off UV and pull carbon while it works. If the tank has no biofilter and needs its daily change to hold ammonia at zero, keep making it and top the dose back up in proportion to the water you took out.
One warning if you do end up treating: formalin and malachite products such as ProForm-C are labelled for use only below 0.05% salt. If you need one, drop the salt with successive water changes first, and run every airstone you own. Formalin strips dissolved oxygen, and warm salted water holds less of it to begin with.
The rest waits for a diagnosis. Formalin and malachite products are hard on gills and on your filter, and a freshly shipped fish is already immune-suppressed. Potassium permanganate is effective and unforgiving; misjudge the organic load and you burn gills. Blanket dosing also masks the symptoms you are watching for. Diagnosis means a skin scrape: run the edge of a glass slide along the flank and over the gill cover, add a drop of tank water and a cover slip, and look at 100x and then 400x. Most koi clubs have a member with a microscope.
One correction worth stating plainly: MelaFix is a tea-tree oil antiseptic, not an antibiotic. It is fine as mild support for a minor scrape, and it will not cure a genuine bacterial ulcer. Treating one with it wastes the week where the fish still had a good prognosis. A spreading or red-margined ulcer needs cleaning, a topical, and often an injectable antibiotic from a vet. Stock a shelf from the koi medications range, with the diagnosis first.
The four-week quarantine protocol
Week 1: intake, acclimation, and the shipping water
Float the sealed bag in the tank for 20 to 30 minutes with the lights low to equalise temperature. Then move fast: open the bag into a clean tub, add tank water gradually over 20 to 30 minutes, net the fish across, and pour the shipping water down a drain, never into the tub or the pond.
Speed matters because of chemistry. In transit the fish exhales CO2 into a sealed bag, CO2 forms carbonic acid, and bag pH drops, often below 7. At low pH the ammonia the fish excreted stays locked up as ammonium, which is comparatively harmless. Open the bag and the CO2 gasses off within minutes, pH climbs back toward normal, and that ammonium flips to free ammonia, which burns gills. A koi that was fine at hour 30 in a sealed bag can be in trouble twenty minutes after you open it, which is why long drip acclimations are a bad idea for shipped fish.
After release: dim light, no food for 24 hours, a token feed on day three, and salt to 0.3% across days one to three. Test ammonia and nitrite daily with an API Freshwater Master Test Kit and water-change on any reading above zero.
Week 2: observation and the fluke course
Watch at feeding time. A healthy koi comes to food, holds position without leaning, and breathes steadily. Flag flashing, clamped fins, sitting on the bottom, rapid gill movement, excess mucus, raised scales, or any red streak or open lesion. Dose praziquantel this week and repeat three days later, holding off on water changes for seven days from the first dose. If a symptom appears, treat that condition and nothing else: a dedicated fluke treatment for a heavy fluke burden, an ich product for white spots, a proper antibacterial for a diagnosed ulcer.
Weeks 3 and 4: unmedicated observation
These two weeks are the point of the exercise. No medication unless something appears. Change 10 to 20% of the water weekly, or daily if the tank has no biofilter. Test ammonia and nitrite twice weekly once the tank is stable, and check pH and KH weekly with a GH and KH test kit. In the last week, step the tank temperature down toward the pond at about 2°F per day: a 76°F tub and a 64°F April pond means six days, not an afternoon.
Release criteria
- Normal appetite and normal activity for 14 consecutive days.
- No medication for at least the last 7 days, ideally 14.
- No spots, ulcers, cloudy eyes, fin damage, raised scales or mucus patches.
- Ammonia 0 ppm and nitrite 0 ppm on the last three tests.
- A clean final skin scrape, if you own a microscope.
- Tank within 2 to 3°F of pond temperature and within about 0.4 of pond pH.
Net or bowl the fish across with as little water as possible, and never pour quarantine water into the pond. Then leave the pond alone for a few days: no netting, no big water change. The Introducing New Koi Fish To Your Pond page covers the release side.
Biosecurity: nets, buckets, hands
Label the quarantine net, bowl, bucket and thermometer and keep them away from the pond. If you own one net, disinfect it in bleach solution and dry it fully between systems: a damp net carries parasites and virus particles for hours. Service the quarantine tank last, never first, wash hands and forearms between systems, and drain the tub to a lawn or a sewer. KHV stays viable in water for several days, long enough for one bucket to undo four weeks of work.
Sentinel fish, and whether to use one
A sentinel is a cheap fish kept with new arrivals to see whether it gets sick, on the sound logic that a susceptible fish shows disease before an asymptomatic carrier does. Koi are social, so a cheap companion is reasonable and doubles as a sentinel, bought knowing it can never join the main pond. Buying a fish with the plan of watching it die is something else, and unreliable anyway: resistance varies, goldfish carry KHV without symptoms, and a cold sentinel tells you nothing.
What to buy for a quarantine setup
| Item | Spec to look for | Rough cost (USD) |
|---|---|---|
| Tub or tank | 150 gal, 24 in deep minimum | $100 to $400 |
| Air pump and airstone | Linear diaphragm pump, one or two stones, continuous | $120 to $300 |
| Liquid test kit | Ammonia, nitrite, pH, plus a separate KH kit | $50 to $65 |
| Dechlorinator and conditioner | Chlorine and chloramine removal, ammonia detox | $19 to $85 |
| Salt | Non-iodised, no anti-caking agents, 2.5 lb per 100 gal | $10 to $25 |
| Praziquantel | 1 g per 100 gal, a little over 2 ppm, repeated at 3 days | $30 to $60 |
| Heater and thermometer | About 5 W per gallon, so 500 to 750 W, guarded, plus an independent thermometer | $60 to $150 |
| Net or mesh lid | Weighted, covering the whole surface | $25 to $60 |
A full setup lands between roughly $450 and $1,100, reusable for every fish you buy for a decade.
Frequently asked questions
Do I still need to quarantine koi from a reputable dealer?
Yes. Screening lowers the odds, it does not remove them, and shipping stress is its own risk factor. The Live Arrival and 7-Day Health Guarantee also asks for water parameters with any claim, which is easier to supply from a tank you test daily.
Can I quarantine in a net pen inside my main pond?
No. A net pen shares water with the pond, so parasites, bacteria and virus particles move freely both ways. It isolates the fish and not the disease. A floating pen is only useful to protect an injured fish that already cleared quarantine.
How many gallons does one koi need?
Plan 250 gallons per adult koi in an outdoor pond, so 1,000 gallons suits four grown fish with strong filtration. One inch of fish per 10 gallons works as an interim figure under 12 inches, but size the pond for the adult: a 12-inch koi is nowhere near finished growing.
How often should I test the water in quarantine?
Ammonia and nitrite daily for the first two weeks, then twice weekly once both hold at zero, with pH and KH weekly. Test more often after a treatment, since several medications knock back filter bacteria and an ammonia spike often follows a dose by 24 to 48 hours.
Does salt kill flukes?
No. Flukes tolerate 0.3% easily, and the concentrations that do affect them belong in a short bath rather than a tank treatment. Salt handles several protozoans. Flukes need praziquantel.
If you only do three things
Build the tank before you buy the fish, hold it at 74 to 78°F for the full 28 days, and keep the quarantine equipment away from your pond. Those three cover most of the risk. Once the fish is cleared, our guide to setting up the pond itself covers the system that keeps it healthy.