Throughout the day and night, the water parameters can sometimes adjust just ever so slightly. By testing all of your water parameters on a periodic check, you can make sure that everything is still inline, and if not, do a water change to adjust the water chemistry back to where they belong. Since every tank is different (you would hopefully want to test a nano tank more than a 150 gallon for example), the water parameter checks can be done differently for everyone. What is your water parameter check schedule, and do you do it more frequently if you notice a fish is not up to it's normal self?
I probably would say that I tend to test my water parameters once a month (unless keeping a very sensitive fish species such as Discus or so). Otherwise I would only test it if I noticed that something was off with some of the fish.
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