Koi pond quarantine. Should I quarantine my fish?

Q: I have inherited a Koi pond and I now want to buy some Koi of my own. What procedures do I have to follow when introducing these new residents? Should I quarantine them first? What is the best way to do this? What sort of quarantine facility should I use? And for how long should they remain in quarantine?

A: Firstly, many in the koi trade differ on their views about quarantine. It is defined as an ‘enforced isolation or restriction of free movement imposed to prevent the spread of a contagious disease’. The term quarantine actually originates from its description of the 40 day period considered necessary for effective screening against the Black Death. This approach to quarantine works well with warm blooded animals as they have predictable body temperatures with known and repeatable incubation periods. But it is not an accurate term to use with fish.

When using the term quarantine in relation to koi, we know what we are implying; saying that by virtue of the fact that a koi has been in isolation for a prolonged period and has not shown any outward signs of disease, then it is likely to be free of disease. This, unfortunately, is not true.

The recent incidence of KHV in the koi trade has focused dealer’s minds on quarantine issues. This has highlighted that typical quarantine (a period of isolation) is ineffective at screening against KHV. This is because infected koi can be long-term carriers without showing signs of ill-health or disease.

So rather than use ‘quarantine’ (and be lulled into a false sense of security by simply following a prolonged period of isolation), ‘acclimatisation’ or ‘rest period’ are more accurate descriptions to use when koi are held in an in-between facility, in complete isolation. Even by changing the name to acclimatisation, such a period will still not guard against KHV, requiring us to adopt what has been described as ‘preventative acclimatisation’ to guard against this new virus. This will involve:

  1. Effective isolation of the koi
  2. Raising the temperature to 23-28C for 2 weeks minimum to try to provoke a viral response.
  3. Then add a further period of stress-free acclimatisation involving matching the two systems’ water temperatures prior to moving your new purchases into your main pond.


If you are going to invest in a ‘quarantine’ system, then you should use it in any way you can to screen against all possible pathogens, including the very real threat of KHV, which essentially means adopting the above regime.

What is the best way to quarantine my newly purchased koi?

For the threat of KHV, you will have to follow steps 1-3 above. This regime will also give you the opportunity to screen your koi for more widely occurring pathogens that are visible under a simple light microscope. By raising your system’s temperature to 23C for 2 weeks, you will give any of these more common pathogens a real opportunity to bloom and multiply, but equally raise the koi’s metabolism and immune system so that they can resist the onset of disease. This is particularly pertinent with bacterial problems that are more difficult to assess under a microscope.

What sort of quarantine facility should I use?

An ever increasing number of koi keepers quarantine their new purchases. Consequently, some very nice compact quarantine systems are available off the shelf. A system will comprise a deep holding tank with an adjacent filter, integrated in the design. If you choose to design and build your own, make sure you do all you can to make the pond as stress-free as possible. Inevitably, it will be smaller than your main pond, but by keeping it away from slamming doors and other external stressors you can make it a calming, non-threatening environment.

Your main pond and ‘quarantine’ systems should be far apart with as many doors and walls between them as possible to reduce the risk of cross-contamination. The quarantine system should have its own nets and ancillary pond equipment (bowls, air stones etc)

How long should they remain in quarantine and what do I do while they are in quarantine?

You should suspect the worst for each new koi that you buy, assuming that it is likely to have KHV. In that way, your minimum quarantine period should be a three week regime as described earlier. If you choose not to quarantine for this period then you are running the risk of introducing disease in your main pond. If you’ve invested in a system, then use it to its most effective limit. Otherwise it’s a little like investing in a top of the range home security system, only to choose not to use it once (when you are nipping out to the shops for 5 minutes) – That’s the time you’ll get burgled!

There are differing views on how quarantine should be managed. Many advocate the use of a permanent salt treatment in the pond as a stress and disease-reducing treatment. Others will take this further and dose the pond prophylactically with anti-bacterial and anti-parasite treatments, irrespective of the results of a mucus scrape.

I believe that treatment should only be added to the quarantine pond in response to a specific, identified pathogen (recognising that treatments that are needless will stress your koi unnecessarily).

What if I find a pathogen on the mucus scrape?

You will nearly always find some level of parasitic infection on a mucus scrape – this is natural. You should treat if the density on your sample is unnaturally or worryingly high, or if you koi are showing obvious signs of disease-related stress (clamped fins, gasping, heavy mucus coating etc).

Box Out: 10 Facts to remember when introducing a new pond resident.

1. If you haven’t got an isolation system – think seriously about installing one.

2. Be aware that a period of prolonged isolation does not necessarily constitute effective quarantine.

3. Assume every new koi purchased could have KHV and acclimatise accordingly

4. Make the isolation system as stress-free as possible

5. Position your isolation system as far away from your pond as possible, with as many doors and walls between the two.

6. Screen for more common pathogens, particularly parasites, using a microscope.

7.  Treat when there is an obvious excessive loading of parasites, or associated symptoms.

8. Keep the isolation system covered as newly-introduce koi will often jump clear of the surface.

9. Keep a diary of all new purchases, treatments, observations etc

10. Quarantine every fish, even if it has been sold to you as ‘quarantined’. The one time you choose to take a short cut will be the time you get caught out.

Kill blanketweed and string algae.