This will lower the pH (i.e., the water will become more acidic).
KH is a measure of dissolved carbonate and bicarbonate salts only. Understanding KH is especially important in planted tanks, as carbonates help a tank resist swings in pH and are commonly known as pH buffers. In planted tanks, plants photosynthesize during the day and consume CO2, which can be dissolved in water as carbonic acid. The consumption of this acid can cause the pH to rise (i.e., the water will become more basic). At night, plants respire as animals do, and along with the livestock and bacteria, will also add CO2, thus more carbonic acid, back to the system. This will lower the pH (i.e., the water will become more acidic). These swings in pH can harm fish and invertebrates, increasing their susceptibility to disease over time.