Old pond syndrome is not a recognized term, but the problem is. This can occur when koi are kept isolated by themselves in a pond for more than 8 to 10 years. No new fish of any kind are introduced to the pond. What seems to happen is that the koi’s immune system begins to weaken. Because the koi have not been exposed to any new viruses, their immune system becomes inadequate. Then, when a new healthy koi is introduced to the pond, the existing koi’s immune system gets overloaded, often causing death.
The first time I saw this was seven years ago. A customer had too many koi in her pond and wished to sell some of them. I purchased two beautiful koi from her, brought them home, and promptly placed them in my quarantine tank with two small healthy koi who had been in that tank for some time. Within three days, both of the new koi became lethargic, laid on the bottom of the tank and died. I tested everything I could think of that might have contributed to their death. I found nothing: no parasites or sores. The existing two koi stayed perfectly healthy. Three months later, she contacted me and wanted to sell her beautiful showa. I told her what had happened to the last two koi I purchased. She asked me to take this koi and see what happens. I placed this koi in a different pond with different koi, but in three days it also died. It turned out that she had a pond full of koi that she could never sell. To make matters worse, she could never add a new koi without losing them all.
We at Sacramento Koi have noticed that typically when a new koi is introduced to a pond, the existing koi become very sluggish for some time. Then after several days, they all perk up again. It appears that the new koi introduce new viruses (like a cold germ to us), but after a few days the koi do recover. Typically this is not an issue. However, the problem arises when koi have been isolated for many years. When new koi and viruses are introduced, their immune system cannot handle it.
What can you do with this information? Anytime you purchase koi from a private collection, be careful. If they have not added a new koi for several years, you might wish to bring up the possibility that their koi might be too immune-deficient to survive elsewhere. At that point, the two of you might talk about how to handle that possibility. If purchasing a whole collection, you might take one or two of the least expensive koi and place them in a tank with one of your koi for one week. You will know if you have a problem within one week. How do you prevent this from happening to your collection? Do not go more than five years without introducing at least one new koi to your collection.