Quarantining Pond Fish: Set Up the Tank and Follow the 4-Week Protocol

Everyone has heard that new fish should be quarantined. Far fewer people know how to do it properly. A bucket of tap water in the garage for eight days protects against nothing; a properly set-up tank at the right temperature for four weeks protects against almost everything. Quarantine is not a formality: it is a small pond in its own right, with its own filter, aeration and heater, and a precise observation schedule. Here is how to set it up and manage it, from equipment to reintroduction.

Why quarantine saves entire ponds

A fish that arrives at your home has undergone transport, handling, a water change and often several successive sales tanks. This stress brings hidden problems to the surface: parasites present in small numbers, an early bacterial infection, or sometimes a latent virus. If the fish goes directly into the main pond, everything it carries reaches your other fish at the same time.

Quarantine reverses the balance of power. For four weeks, the newcomer lives alone (or with the other newcomers) in a small volume that is easy to observe and treat. A problem that appears then affects one or two fish in a few hundred litres, rather than an entire pond holding several cubic metres, including the filter. Treating a tank costs little; treating a large pond is expensive, time-consuming and less successful.

Quarantine is also for fish that return: a koi that was lent out, displayed at a competition or treated elsewhere goes back through the tank before rejoining its companions.

Size the tank according to the fish

The tank must provide good living conditions for a month. If it is too small, it becomes unstable: the slightest feeding error causes ammonia levels to rise. The general guidelines we recommend are:

Choose a round or oval tank with smooth walls, in a light or blue color so you can see the fish clearly. Rigid tanks sold for fish farming or removable above-ground ponds are well suited. Install it away from direct sunlight and frost, in a quiet, accessible location with an electrical outlet protected by a residual-current device.

Equipment: mature filter, aeration, heater, cover

• A mature filter, meaning one colonized by the bacteria that convert ammonia and nitrites. This is the point most often overlooked. The simplest solution: keep a piece of foam or a bag of filter media permanently in the main pond filter, and transfer it to the tank filter when needed.
• Permanent aeration: air pump and diffuser, even if the filter is already circulating the water. Warm water contains less oxygen, and a stressed fish consumes more. Our article on choosing an air pump for koi helps you size it correctly.
• A heater with thermostat, to keep the water stable at around 20 to 24 °C, whatever the season.
• A cover net: a stressed fish may jump, and an uncovered tank is an invitation to cats and herons.
• A thermometer and test kit (ammonia, nitrites, pH, hardness).
• Dedicated equipment: net, bucket, gloves, used only for the tank.

Prepare the water and mature the filter

The tank should be prepared several days before the fish arrive, never on the same day.

1. Fill the tank with tap water treated with a conditioner, or with a mixture of water from the main pond (if it is healthy) and treated fresh water.
2. Start the filter, aeration, and heater, and allow the temperature to stabilize.
3. Seed the filter: transfer mature filter media from a healthy pond or, failing that, use starter bacteria according to the instructions and allow time for the filter to establish.
4. Test the water: ammonium and nitrites must be zero, and the pH and hardness should be close to those of the main pond.
5. Record the initial values on your monitoring sheet.

If you start a filter from scratch without mature filter media, allow several weeks before it becomes fully effective: in this case, greatly reduce feeding and increase the frequency of partial water changes during quarantine. Our article on starting a new pond explains this cycle in detail.

Weeks 1 to 4 protocol

The schedule below is the one we recommend for koi. It can be adapted for other pond fish.

Golden rule: any sign of illness, treatment, or death during quarantine resets the clock from the end of the episode. It’s frustrating, but that is precisely the purpose of quarantine. If a fish dies, have it analyzed promptly rather than assuming it was an accident: this is often the only way to find out what is happening.

Temperature deserves particular attention. If your fish arrive from cool water, raise the temperature gradually, by about one degree per day, until reaching the target range. Quarantine conducted at 10 or 12 °C does not reveal latent diseases, including certain viral diseases: it creates a false sense of security.

Salt baths and preventive treatments: when yes, when no

Salt is the most widely used supportive tool in quarantine. At a low concentration and over time, around 1 to 3 grams per liter, it helps fish regulate their water and mineral salt balance after the stress of transport and hinders certain parasites. Short baths at higher concentrations require a precise protocol, described in our dedicated article. In all cases, salt should be dosed based on the tank's actual volume, measured rather than estimated, and dissolved before being added.

What we advise against: cocktails of antiparasitic and antibacterial products added “as a precaution” as soon as the fish arrive. They stress the fish, disrupt the filter, and can mask useful diagnostic signs. Treatment should be given based on observed signs or, better yet, after a microscopic examination. To learn about the main diseases, see our guide on koi carp diseases.

Observe and record: the monitoring sheet

Quarantine without notes is only half done. Our memory is poor at details: “I think it was already rubbing itself on Tuesday.” A simple sheet, filled out every day, makes it possible to spot changes and provide precise information to a professional if needed.

Always observe at the same time of day, and spend a few minutes staying still before looking: a fish that hides at the slightest movement will not show its normal behavior. A weekly overhead photo also helps detect changes in the skin or fins.

Reintroduction into the main pond

After four weeks without any signs, the time has come. A few precautions will prevent everything from going wrong at the last moment:

1. Equalize the temperatures: if the tank is at 22 °C and the pond is at 15 °C, gradually lower the tank temperature over several days. The difference should be no more than a few degrees at the time of transfer.
2. Check the main pond water before transferring the fish.
3. Transfer the fish alone, using a net or fish sock, and never pour water from the tank into the pond.
4. Monitor the main pond for two to three weeks: a problem may still appear, either in the newcomer or in the existing fish.
5. Clean and dry the tank equipment, or leave the tank running empty if you plan further introductions.

A common scenario in consultations: an enthusiastic owner buys fish every spring and eventually installs a permanent tank, with its filter always running and mature media kept in reserve in the main pond. Introducing new fish has become a stress-free routine, and unpleasant surprises have disappeared. That is exactly the goal: make quarantine a reflex, not an exceptional inconvenience.

Read also

A salt bath is one of the most useful measures during quarantine: How to give fish a salt bath during quarantine?.

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Frequently asked questions

— How long should quarantine last for pond fish?

Four weeks is a reasonable minimum for koi, at a temperature of approximately 20 to 24 °C. Any sign of disease or mortality resets the clock.

— What size tank is needed for koi quarantine?

Allow at least a few hundred litres for small fish, and preferably 1,000 litres or more for large koi, with sufficient depth and smooth walls.

— Should fish in quarantine always be treated?

No. Salt can be used as supportive care, but antiparasitic or antibacterial treatments should be based on observed signs or a diagnosis, not administered blindly.

— Can quarantine be carried out in winter?

Yes, provided the tank is heated. Cold-water quarantine does not reveal latent diseases, particularly viral diseases, and gives a false sense of security.

In summary

• The quarantine tank is a complete mini-pond: sufficient volume, mature filter, aeration, heating, and cover.
• Target temperature of around 20 to 24 °C for four weeks to reveal latent problems.
• Daily observation, regular water tests, partial water changes, and a written monitoring log.
• Treatments only based on signs or a diagnosis; salt can be used as supportive care.
• Reintroduce at the same temperature, use separate equipment, and monitor the main pond during the following weeks.

Last updated: October 3, 2026.