Old pond syndrome is not a recognized term but the problem is. This is where some koi are kept in a pond for more than 8 to 10 years isolated by themselves. No new fish of any kind were introduced to the pond. What seems to happen is that the koi’s immune system begins to weaken. Because they 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 broadsided and they die.
The first time I saw this was 7 years ago. A customer had too many koi in her pond and wished to sell some of them. I purchased two nice koi from her, brought them home and promptly placed them in my quarantine with two small healthy koi who had been in that tank for some time. Within 3 days both 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 and found nothing. No parasites or soars. The existing two koi stayed perfectly healthy. Three months later she contacted me and wanted to sell her beautiful showa. I told her that I had not had much success with the last two koi I purchased but did not know why. She asked me to take this koi and see what happens. I placed this koi in a different pond with different koi and in three days it also died. It turned out that she had a pond full of koi that she could never sell. Additionally, she could never add a new koi without the risk of loosing them all.
Because of this poor economy many people have been forced to sell their homes. This has created a market flooded with koi for sale. Two times in the last 18 months I have seen individuals who purchased a whole collection of koi watch them all die within one week after introducing them into their existing koi pond. In both cases, the purchased koi had been isolated for over 10 years without being exposed to another koi.
I have noticed on several occasions when we introduced a new koi to one of our customer’s ponds the existing koi became very sluggish for several days afterwards. Then after several days they all perk up again. It appears that the new koi introduced a new virus (like a cold germ to us) and it took a few days for the koi to recover. The problem is when they are isolated for years and then get hit with a bunch of viruses all at the same time. It becomes too much for them to handle.
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 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. I know coming from a koi dealer that this sounds like a sales pitch but the problem is real.