A Koi that looks perfectly healthy can be carrying a virus which may take more than a week before it starts showing signs of illness. In a worst case scenario, where the fish is carrying a potentially deadly virus like KVH (Koi Herpes Virus) or SVC (Spring Viremia of Carp), quarantining may actually save your existing stock from being completely wiped out.New Koi fish should be quarantined for a period of three weeks and their health closely monitored during this time. Once this period is over and you are confident that they are healthy, they can then be safely introduced to your pond.
Take care to acclimatise the fish slowly to their new home in order to minimise stressing the fish. Because the temperatures of the water in the quarantine tank and pond are likely to be different, you should bag your Koi first of all with some of the tank water and use a rubber band to fasten the bag. Let the bag float on the surface of the pond for 15 to 30 minutes to allow the temperatures to equalize. Do not let the sun overheat the bag so make sure you keep an eye on it.
After this time, open the bag and gradually fill it with pond water a little bit at a time. Repeat this step about 4 to 5 times in 10 minute intervals. This will allow the new Koi to adapt to the chemistry of the pond water like its pH, alkalinity etc. Avoid emptying the bag water into the pond though - just remove the fish out of the bag with a net or pick the fish up with your hands and release it into the pond that way.
Despite these efforts to minimise stressing the fish, moving the Koi will still be stressful for them. These fish may try jumping out of the pond when first released into it so a good idea would be to put netting over the pond to stop them succeeding.
Finally, do not feed your new fish for 24 hours. Give them time to adjust to their new surroundings.
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