How to Quarantine Koi & Other Pond Fish (Step-by-Step)

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How to Quarantine Koi, Goldfish, & Other Pond Fish (Step-by-Step)

Isolated koi fish
If possible, new fish should be quarantined for a brief period before being introduced into a stocked pond or aquarium. KoiQuestion / CC BY-SA 2.0

Biosecurity is a vital aspect of keeping a healthy pond. When fish are transported from place to place, they can introduce potential pathogens that may wreak havoc on young ecosystems. Even well-established ponds and mature fish can become compromised due to contact with foreign microbes. This is why any new fish should ideally be quarantined before it is released into a stocked pond or aquarium.  

Many ornamental fish species that enter the US and Europe are cultivated in other continents with totally different communities of native microbiota. In those distant regions, the fish may have developed a natural immunity to these microbes, and may therefore show no signs of ill health. It isn’t always possible for hatcheries to detect the presence of these pathogens, and it’s certainly near impossible for buyers to make certain all of them are absent in the fish that they buy.

The most prudent means to reduce the chances of an outbreak is by observing a brief quarantine period. This involves caring for the new fish in a totally isolated setup just until you can be certain they are indeed healthy and disease-free. If this step is skipped, fish importers run the risk of spreading dangerous pathogens into shared water systems. This may eventually result in massive fish die-offs in both public and private waterways. This is why responsible fish importers always quarantine their fish.

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Which Fish Need to Be Quarantined?

Caught fish
It’s not recommended to source fish from the wild as they may introduce diseases to your pond. Photo from pixabay

Most of the time, fish purchased from reputable hatcheries and fish shops are free of dangerous pathogens. If you’ve repeatedly purchased your fish from local aquarists and have not observed any symptoms of disease, you may simply acclimate their fish prior to releasing them into your pond. Local fish stores will have likely quarantined any imported fish before they put them on sale.

If you’re waiting on fish from distant areas, quarantine should absolutely be a part of your reception protocol. Some freshwater fish species are more disease prone than others and may need lengthier quarantine periods. Others may have been reared in certifiably pathogen-free hatchery systems, and these may require less rigorous and costly quarantine methods.

If you intend to introduce wild-caught specimens into a well-established pond, the quarantine step is also a must. Sourcing fish from the wild is largely discouraged as they are likely to be vectors of parasites and water-borne diseases. Hatchery-grown fingerlings can become immunocompromised when exposed to wild-caught fish.

Note that the water that comes with the imported fish carries a foreign microecology. Even if the species that you purchase is not prone to spreading common diseases, they may be transported in contaminated water. The most effective quarantine methods are those that identify all points of possible cross-contamination.

Setting Up the Quarantine Facility or Tank

Fish tank
If you don’t have a glass tank to quarantine fish, you could use a water trough, show tub, plastic drum, or even an inflatable pool! Jurassic Blueberries / No copyright

All fish owners rearing considerable quantities of high-value fish should have a spare tank for quarantine purposes, for rearing fry or juveniles, and for isolating and treating potentially infected fish. A series of tanks or a full-fledged quarantine facility will be required for large-scale importations. Listed below are some of the basic requirements of every effective quarantine setup:

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The tanks or drums used don’t have to be top-of-the-line or anything fancy, they just have to be reliable. In place of glass tanks, you can make use of sturdy plastic drums, water troughs, show tubs, and even inflatable pools. They should be large enough to provide each fish with a generous amount of space.  Make sure these have been thoroughly cleansed of any chemicals or food-based substances. Ideally, their internal structure should also allow for efficient cleaning.

The tank should be equipped with an adequately-sized water pump and filter. Keep in mind that the quarantine procedure should prevent the new fish from becoming increasingly stressed after their long journey. The water quality should be optimized so the fish can adjust to their new environment in comfort. Fish importers will usually situate the quarantine setup in a dark area as bright light may aggravate stress.

All tubing materials used for the setup should be thoroughly sterilized and checked for holes before use. An air pump that delivers clean air to air stones should work quite well for quarantine tanks. You may eventually find that some fish will opt to stay close to the bubbles as they adjust.

Lastly, ensure that each tank is equipped with a porous cover. This is vital as stressed fish scare easily and may attempt to jump out. Fear isn’t the only cause of attempted escapes; poor quality may also result in the occasional jumper. Regularly monitor water parameters to ensure that the fish remain calm and comfortable throughout a quarantine period which may last anywhere from 2 – 4 weeks or more.

Quarantine Procedure

This outlined procedure is intended for use as a general quarantine guide. In the event that your fish arrive from unlicensed facilities, are wild-caught, or begin to show symptoms of infectious diseases and parasites, you may need to adopt stricter quarantine measures and make use of approved medications/additives (e.g. broad-spectrum antibiotics, salts, antifungal & antiparasitic medication) to condition the water. Some issues cannot be fixed with a simple quarantine procedure, no matter how long it lasts. Consultation of a local expert is recommended in extreme cases.

1) Pre-reception (Day 0)

Salt crystals
Adding low concentrations of pure rock salt or aquarium salt to your tank can reduce fish stress levels and speed up the adjustment period. Nate Steiner, CC0, via Wikimedia Commons

A few days or weeks prior to the expected date of fish delivery, you will have to inspect each component of your quarantine setup. The flow of water and air into the tank should be seamless. Electrical circuits should be double-checked and at least one backup energy source should be made available in case of power disruptions. Expect to receive stressed, disoriented, and low-energy fish that will require perfect water conditions to become well-adjusted.

If you wish to introduce beneficial bacteria into the system’s filter, this must be done well in advance so that their colonies can take hold. Pure rock salt or aquarium salt, without any additives, may also be added at a concentration of about 0.1 – 0.5% of the volume of water. This low salt concentration should help reduce stress levels in the tank and hasten the adjustment period.

The tank(s) should already be filled with aerated water once the fish arrive. The water should ideally be between 74 – 76˚F (23 – 24˚C) or at the temperature prescribed by the hatchery. Each tank should also be equipped with secured netting. Conduct all necessary tests the day before and the morning of the delivery date. Go over the steps that must be done once the fish arrive so that you won’t waste time speculating on the next course of action.

2) Receiving the fish (Day 1)

Acclimating fish
To acclimate your fish to their new environment, float them in the tank while they are still in the bags that they arrived in. USFWS Mountain-Prairie / No copyright

Once the fish arrive, take the bags out of their boxes and float them in the tanks. Dim the lights so that the fish don’t become stunned. It will take at least 15 – 20 minutes for the water temperature in the bags to be identical to that of the tank. This acclimation step helps prevent thermal shock, which can significantly weaken ornamental fish.

Once the temperatures are identical, carefully open the bags and release the fish. Take care to minimize the amount of water released from the bag. Both the water pump and the aerator should be working continuously at this point. Replace the cover of the tank and observe the newly released fish for a few minutes. Avoid making any abrupt movements or loud noises that may scare them.

The fish are likely to swim toward the bottom of the tank, close to the air stones, and settle there for the rest of the day. If all seems to be functioning normally and the fish are calm, dim the lights and leave them be.

3) First feeding (Day 2)

Fish food
You can try to feed your new fish if they show signs of activity, such as swimming closer to the surface. Tommy Wong / CC BY 2.0

The fish may still appear to be largely inactive the following day. It’s normal for them to be extremely wary and timid within the first 24 – 48 hours. If they do show signs of activity and have begun swimming closer to the surface of the tank or exploring their surroundings, you can try to feed them. Make sure to provide high-quality fish food that is suitable for their size and species.

Begin by dropping just a few pieces into the water, and wait to see if there are any takers. If there are, add just enough food so that the feeds are completely consumed in a few minutes. If the fish have no appetite, let them rest and try feeding them again another day.

4) Normalizing a routine (Day 3)

You can begin performing small water changes around day 3.

At this point, the fish should be quite rested and more active. Try to observe them from a distance as they are more likely to explore the tank when they are unable to see any spectators. Begin normalizing your feeding routine once the fish readily take feeds. As a precaution, you can feed them with medicated feeds. Try feeding minimal amounts at a higher frequency until you are able to determine the right routine. This will largely be affected by the fish’ energy levels throughout the day, along with their consumption capacity.

At this point, you can also begin performing water changes. If minimal waste has been produced in the past 2 days, a 10 – 15% water change should suffice. You should aim to change at least 25% of the water volume in a single week. The waste materials can also be siphoned out of the tank to prevent unnecessary contamination.

Healthy specimens should normally be able to recover from stress during the first few days of quarantine. Exceedingly stressed, immunocompromised fish may perish despite optimal water conditions. If more than just 1 or 2 fish die, the quarantine tank may require medication.

5) Next 2 weeks (Day 4 – 14)

Water testing
From day 4 to 14 you should be regularly checking water parameters. Department of Foreign Affairs and Trade, CC BY 2.0, via Wikimedia Commons

Continue performing frequent water changes and monitoring water parameters. Normalize your feeding schedule with the goal of providing feeds at the same times each day. If all appears well, you can transition back to using high-quality fish feeds instead of medicated ones. If your setup is in an area that receives bright and natural light, the fish should be able to tolerate full day lengths once they regain their energy.

If you notice any signs of parasites and disease symptoms, you may need to medicate the water with a broad-spectrum antibiotic or anti-parasitic solution. During the medication period, which may last for a full week, you will need to stop feeding your fish and conducting water changes. The fasting period can be reduced to 4 or 5 days if your fish show signs of hunger.

Some aquarists prefer to treat their fish during quarantine even if there are no signs of illness. They do so particularly if they have high-value specimens in the ponds that will eventually house the new fish. Others wholly avoid using medication, as it is essentially toxic, unless absolutely necessary.

6) Prolonging the quarantine period (Week 3 & up)

Koi herpesvirus
Koi herpesvirus can take at least 3 weeks to appear in koi. Photo from

The state of your fish at the end of the 2-week period determines their readiness to be introduced into the pond system. If you’re still worried about the impact they may have on your pond fish and vice versa, you can actually take a single fish from the pond (preferably the least valuable one) and introduce it into the tank system.

If the pond fish can easily adjust to the tank system and thrive alongside the new fish, it is likely they are now compatible enough to be in the same body of water. If a fish shows unhealthy symptoms soon after the introduction, you may need to extend the quarantine period to 4 weeks or more. Note that some diseases, such as koi herpesvirus, may take at least 3 weeks to manifest.

Some aquaculture labs and freshwater fish research centers may have tests for a handful of infectious diseases. If you’re able to purchase these, you’ll find that they are often worth it when dealing with high-value species such as purebred strains of jumbo koi from Japan. If a fish tests positive for any diseases, do not introduce it and any accompanying fish into the pond any time soon. If all the fish are healthy and active at the three-week mark, they can finally be released into the pond.

How to Introduce Quarantined Fish Into the Pond

Fish being introduced to pond
It should be safe to release your new fish into the pond after acclimating them for a few minutes. Oregon Department of Fish & Wildlife / CC BY-SA 2.0

The fish should again be acclimated before they are released into the pond water. The temperature difference between the tank and the pond should be very minimal. You can either place the fish in oxygenated plastic bags and float these on the pond, or you can acclimate them by gradually transferring more and more pond water into the tank. After a few minutes, it should be safe to release them into the pond.

Closely keep an eye on both your older pond fish and the newly introduced ones over the course of the next 2 – 3 weeks. New fish will likely go through a re-adjustment period, causing them to be somewhat inactive during the first 1 or 2 days. After this period, however, they should begin to explore the pond and readily take food.

Common Diseases in Imported Fish Stocks

Fish lice
Fish lice are one of the most common parasites in imported fish stocks. Des Colhoun / CC BY-SA 2.0

In general, infectious fish diseases can be categorized based on their source. There are locally-occurring diseases, which are caused by indigenous pathogens, and there are exotic ones, which are frequently considered “high-risk”. Always purchase your fish from reputable dealers that can reassure you about the quality of their fish or can test for the presence of pathogens prior to shipment.

Local pathogens are usually controlled by observing safe management practices along with the use of approved medication. Exotic diseases, sometimes called ‘transboundary fish diseases’, are more threatening and can put entire wild populations at risk. Importers need to be aware of the fish diseases and parasites listed below. These tend to differ across species, and some may be transmitted zoonotically.

If you suspect that any of your fish have symptoms of the listed diseases/parasitic disorders, isolate them and do not delay treatment. Contact a local veterinarian or fish expert for more guidance. Lastly, don’t be too worried in the event that your fish display troublesome symptoms during quarantine. What matters is that you are able to pro-actively respond to potential problems and resolve them before the fish are introduced into your pond.

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