For lack of a better name I call this phenomena “New Pond Syndrome”. What I have noticed over the years is that no matter how well you build a pond, what filter you install on the pond or how closely you monitor the water, the koi are prone to getting sick during the first year. This is not to say that building a great pond, with a great filter and watching the water quality closely does not help. All I am saying is that with a new 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 easily kill some koi before you know it. If any of the koi develop an ulcer, it seems to take longer than normal for the ulcer to cure. Even if you are forced to perform a larger water change than normal, due to high ammonia or nitrite build up, the koi seem to get overly stressed. It can be very frustrating and a real heartache to a new hobbyist. The good news is that it goes away after the first year.
I have spoke to several veterinarians about this phenomena and they all agree it is a real problem. 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 which usually takes at least 6 weeks. What we sometimes overlook is how long it takes for the pond itself to mature. We often get concerned about the lye from a concrete pond or from the rockwork around a liner pond leaching into the pond. This lye can cause a very high ph reading in the pond water. The problem is that once we get the ph down to a manageable level we start feeling everything is perfect. There are some, including myself, that feel very strongly that there are other chemicals being released in such small quantities that they are very hard to measure but are very stressful to the koi. Once the pond matures and a healthy layer (¼ inch) of algae covers all the surfaces of the pond, 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 way down. When they are no longer stressed they then have the ability to 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. Be careful where you get your koi from to prevent introducing parasites. Your friend may want to give you some of his excess koi but you need to realize that his pond is 10 years old. His koi could easily have a low level of parasites but are able to function well because they are not under stress. Healthy koi, that are not stressed, are often times able to fight off the parasites due to a healthy slime coat. When they get into your new pond and become stressed they may soon loose 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. After the first year all this will go away and then you can relax