Water quality is to Koi as air quality is to humans. This is why Koi water quality has more impact than anything else on your fish's overall health.
Low oxygen in a pond can kill every fish in the pond literally overnight. High ammonia levels can kill fish within several days. But, even borderline levels of oxygen, ammonia and nitrates can impact your Koi's immune system enough that they contract other life threatening problems.
Pond water conditions can vary significantly on a day-to-day basis. This is especially true when a new pond goes through the initial 'maturing process'.
As a responsible Koi owner you you need to perform frequent "indicator" Koi water tests to make sure the water in your pond is of a sufficient quality to support Koi health. The reason they are called indicator tests is because they are not as accurate as those done in a laboratory setting.
You could send a water sample in to a lab for a complete water analysis but this is generally not economically feasible.
The following are the suggested Koi water tests:
1. Ammonia test - This reading should be zero. Ammonia build-up can be fatal to your Koi. The initial maturing process in your pond can take over six weeks to stabilize and high ammonia readings can be extremely dangerous. Water changes can reduce ammonia build-up and daily tests are extremely important in new ponds.
2. Dissolved Oxygen test - This test is very helpful during the hotter months when high water temperatures can deplete the dissolved oxygen content of the pond water. Keeping track of your pond's temperature via a thermometer is also recommended. Always try to maintain a reading of 6.0ppm and over. If you are having difficulty consider adding aeration with a suitable air pump and air stones and/or installing a waterfall. It is not healthy for your Koi to be gasping for air at the top of the pond!
3. pH test - For healthy Koi water readings, we require a pH reading of between 7.00 to 8.00 although a reading of 7.5 to 7.8 is optimal. In a new pond the pH reading can fluctuate from morning to night in addition to day-by-day. It is advisable to take morning and evening readings and record them every two days until they are stabilized. For mature ponds, generally a monthly or quarterly pH test is all that is required.
4. Nitrite test - This reading should be zero for optimum water conditions. However, it will vary widely in new ponds and can prove to be serious if not addressed promptly. If the reading is high then feeding should cease and constant new water added to the system until the bacterial activity in the filter begins to be able to handle the nitrite build-up. Nitrite readings should be taken and recorded on a daily basis until they are consistent. After that monthly or quarterly tests should be fine.
Although there are also many other types of tests available, these four tests are sufficient to ensure adequate Koi water quality.