There are a few critical reasons. First, while fish can generally adapt to pH levels that are outside of their ideal range, they cannot handle dramatic, rapid swings in pH. We take phone calls every week from customers who are struggling with what appear to be random fish deaths. When we ask them to measure their pH several times during the day, we often see dangerous intra-day swings of an entire point or more! We then ask them to test the KH levels in their water and they nearly always report that they have a dKH that is below 4.0.
Second, maintaining a buffer is critical for bacterial health. If you get to the point where your system carbonates are completely depleted, your system pH can “crash” (decrease rapidly). If it does, your beneficial bacteria will quickly die and biological filtration will stop entirely.
Third, if you don’t have at least a minimal buffer of 4 dKH established in your system, you will need to manage pH on at least a daily basis (i.e. measure it and adjust as needed). If there is no buffer to offset the nitric acid in your system it will simply lower your pH, unimpeded. So not only is a lack of carbonates dangerous to your fish and bacteria, but it will also create a maintenance nightmare for you!