We have noticed over the years that koi are prone to getting sick during the first year of being introduced to a newly constructed pond, no matter how well that pond is built. For lack of a better term, we call this “New Pond Syndrome.” If any of the koi you add have any parasites at all, these parasites will spread like wildfire and can quickly kill some of your koi before you know it. If any of the koi develop an ulcer, it seems to take longer than normal to heal. If you are forced to perform a significant water change due to high ammonia or nitrite build-up, the koi seem to get overly stressed. It can be very frustrating to a new hobbyist. The good news is that it goes away after the first year.
We have spoken to several veterinarians about this situation, and they all agree. The problem seems to be that the new pond has not had a chance to mature. We often talk about how important it is for the new filter to mature, usually taking at least six weeks. The pond itself need time to mature also.
One issue that a maturing pond faces is the releasing of chemicals into the pond. For example, lye is leached into the water due to the concrete in the pond and/or rock work around it. This lye can cause a very high pH reading, which can be dangerous for the koi. Once the pH lowers to a manageable level, some people may start feeling that everything is perfect. However, we feel strongly that other chemicals are still being released into the pond that are very stressful to the koi.
These chemicals are challenging to measure because they are released in such small quantities. Once the pond matures and a healthy layer (¼ inch) of algae covers all the pond surfaces, these trace chemicals seem to go away. Dr. Erik Johnson jokingly calls it “mojo.” He says he can’t explain it, but once the pond “mojo” develops, the stress level of the koi goes down. When the koi are no longer stressed, they can then fight off both parasites and diseases.
What can you do with this knowledge? Work with your pond for the first year. Do not overload a new pond with fish. Try to resist the temptation to put expensive koi in the pond for the first year.
Also, be careful where you get your koi from to prevent introducing parasites. For example, say a friend wants to gift you some of his koi. They seem parasite-free because they are not under stress. However, those koi could easily have a low level of parasites. Koi that are not stressed can often fight off parasites due to a healthy slime coat. When those koi are introduced into a new pond, they more than likely will become stressed and lose the ability to fight off parasites.
Watch your water quality closely. Try to keep your ammonia and nitrites as close to 0 as you can and do not let your pH crash. More than anything else, be patient. A good koi pond doesn’t come overnight.