There are those who never test their water and they will say it is silly to do so. And while I agree a pond can stabilize over time and the test results can become predictable, there is much worth to frequent and regular testing of the water in your koi pond. Even a stabilized pond is always changing. The koi grow, the seasons change, the bio filter ages, oxygen levels change, temps changes, etc. Testing only takes a couple minutes and should be a part of every hobbyist’s weekly routine.
Now, you may not need to test every possible parameter every day, but there are a few key tests you should perform at least once a week.
First test to perform is Ammonia. It is a good test of the general health of your bio filter. If you test ammonia then something is wrong with your bio filter.... maybe you fed more then normal, maybe O2 levels are down, or maybe you are just over stocked. But ammonia is a bit of a conundrum. While you want the tests to read zero, the koi are constantly producing ammonia and you can never reach zero. In fact you don't want to be at a true zero for ammonia. The bio filter will only grow enough bacteria to convert the ammonia present in the pond. If you were truly at zero the bio filter would die off. But ammonia is toxic...right?? Yes, it most definitely is highly toxic to the koi. What we want in the pond are low levels of ambient ammonia being produced by the koi that is quickly whisked off to the bio filter. (Good pond circulation comes into play here) When one says their ammonia tests at zero, what they mean is the levels are below what the test can read, but they should never truly be at zero.
Confused yet? Well next we move on to nitrites. Nitrites are also quite toxic. They are the next step in the conversion of ammonia by the bio filter. The bio filter will grow a second type of bacteria that will convert the nitrites into nitrates. This secondary bacterium is quite fragile and can die off quite easily from frequent water changes, dramatic changes in pH and even a rigorous backwash. So you need to test for nitrites again as a means to monitor the general health of your bio filter.
Lastly we have nitrates. These are non-toxic to the fish and the final product of the bio filter after it has converted the ammonia into nitrites and then into nitrates. I rarely test for nitrates, maybe only 2 to 3 times a season, but I do frequent water changes that will remove nitrates. If your water changes are less frequent, then testing high levels of nitrates will tell you its time to changes some of the water.
After testing for the health of your bio filter, you need to test the stability of the pond. Namely the pH and the kH. The pH is an important number, but what specific number the pH is at is not as important. What you need to monitor is the stability of the pH. The koi can handle a wide range of pH from as low as 7.0 up to 9.3 or higher. What the koi cannot handle are dramatic changes in the pH. So never add any sort of pH up or pH down products to the pond. Chasing some ideal pH is a waste of time and you end up doing far more harm then good. In fact I find I rarely even test for pH anymore. A far more important number is your kH.
The kH is your alkalinity and it is what will buffer and stabilize the pH. The reason I rarely test for pH is because I am not going to do anything to change it anyway, aside from raise the kH if it’s low. So I test the kH frequently, usually 2 to 3 times a week. The reason for the frequency is because the bio filter will constantly consume the alkalinity and lower the kH. And the older, more seasoned your bio filter, the faster it will consume the alkalinity. In fact higher the kH the more effective your bio filter will be. And when the kH drops not only will it hurt the bio filter, but the pH can shift as well and that's when you run into serious problems. If your source water has a high kH then all you need to do to replenish the alkalinity is a water change. However many municipal water sources are lacking in kH and it is necessary to add Sodium Bicarbonate. (That's just good old Baking Soda to you and me.) One pound per 1000 gallons will raise the kH by 70ppm. But it is safe to raise the kH up to 300 or even 400 if you like and if you have a bead filter you want to keep it above 200 as a minimum. Adding baking soda will buffer your pH right around 8.3 and hold it there rock solid.
The only concern with raising the kH too quickly is if you have a low pH AND you test for ammonia, then you need to treat the ammonia BEFORE you raise the kH. The reason for this is at a low pH the ammonia is not toxic to the koi. But as soon as you raise the kH the pH will also rise making the ammonia toxic. So first add your ammonia binder, then add your baking soda. As I said above, the baking soda will stabilize the pH right around 8.3, but if your pH is around 6.8 and you test 2ppm ammonia, well then dumping a bunch of Baking Soda can be stressful to the koi. In this situation I suggest slowly adding the baking soda over several days. Perhaps 1/2 pound per 1000 gallons per day testing and treating for ammonia along the way.