Since the quality of your Koi water is the single most important factor affecting the health of your Koi it is imperative that you take proper precautions in maintaining it.
There are several key elements you will want to keep a very close eye on. We all know that the #1 most important thing to keep tabs on is ammonia, particularly in a newer pond. It takes time for the beneficial bacteria to colonize enough to help keep ammonia levels under control.
As ammonia levels drop another toxic compound will take it's place. Ammonia is converted by the nitrosomonas bacteria into nitrites, which are also harmful to your fish, but not as deadly as ammonia. As ammonia levels drop in your pond, nitrite levels will rise.
Nitrites (NO2) at low levels can stress your Koi, suppressing their immune systems thus causing them to be susceptible to diseases caused by other factors. High levels of nitrites cause skin and gill epithelia damage, which can lead to parasite attacks and/or secondary bacterial infections.
A common symptom of Koi that are suffering from nitrite poisoning is that they will gasp at the water surface and stay around water outlets. For Koi water to be considered nitrite-safe your pond must register nitrite readings of ZERO.
If you get higher readings you can correct the problem by doing partial water changes. Adding salt to the pond (approximately .02%) also makes nitrites less toxic to Koi. Their gills will tend to take up the added chloride ions instead of the nitrite ions. This will help protect your fish from the nitrite ions.
Sooner or later the nitrite readings in your pond will go down. the reason for this is that the nitrites are converted into nitrates by 'good' bacteria (nitrobacter sp). Nitrates are quite a bit less toxic than nitrites, but you should still check them regularly.
If the nitrate levels in your Koi water are allowed to go unchecked your Koi may start to lose their appetites. Nitrate levels should always be kept under 60 parts per million (ppm)/ 60 mg./liter although some Koi experts say that a nitrate level of 100-500 ppm is not harmful to your fish.
Once again, a good solution to higher-than-normal nitrate levels is a partial water change. The use of plants and trickle filters will also help stabilize the nitrate levels in your pond.
Be aware that there are also metals, such as copper and iron, that are toxic to Koi fish. Some possible sources of exposure can be heaters, pipes and even sometimes filters!
Weekly testing for ammonia and nitrite levels is recommended, and in the very early stages of your Koi pond even daily checks are the best way to go.
Once your pond has matured enough and you are getting consistently stable readings you can cut down on frequency. Nitrate level testing can usually be done only once a month. Of course, any change in the filtration system or koi pond itself will require a return to more frequent monitoring until the readings stabilize again.
If you take these precautions you will be assured of the best quality possible for your Koi water.