For lack of a better name, we call this event “New Pond Syndrome.” What we have noticed over the years is that no matter how well you build a pond, the koi are prone to getting sick during the first year. 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 average to heal. If you are forced to perform a significant water change due to high ammonia or nitrite build-up, the koi seem to become 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. What we sometimes overlook, though, is how long it takes for the pond itself to mature.
One issue that a maturing pond faces is with chemicals being released into the pond. For example, lye is leached into the pond due to the construction. This lye can cause a very high pH reading, whether it’s from the concrete in the pond or the rockwork around it. Once the pH lowers to a manageable level, some start feeling everything is perfect. However, we feel strongly that other chemicals are still being released into the pond.
These chemicals are in such small quantities they are challenging to measure, yet they are very stressful to the koi. 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. Say a wants to gift you some of his koi. His koi could easily have a low level of parasites but seem parasite-free because they are not under stress. Koi that are not stressed can often fight off parasites due to a healthy slime coat. When introduced into a new pond and become stressed, they may lose that ability.
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.