The most important thing to understand when looking at Kh (also known as carbonate hardness) or any other measurement, is that fish live so completely in their environment that we need to balance it for them. The term “kh” refers to the amount of calcium in the water. Whether it is hard water (high level of calcium in the water) or soft water (low level of calcium in the water).
If your water tends to have a high amount of calcium in the water and your fish are low kh or soft water fish, then they fish will simply not aborb all the calcium in the water and should be ok. However, if your fish is a hardwater fish and your water is soft or low kh, it will have a hard time thriving or even surviving as it will not be able to absorb enough calcium from the water.
Low kh is hardest on plants, so if your fish are herbivores and you keep plants to feed them, then having low kh could in the end be hard on your whole tank.
Its a tricky one. Especially for us in soft water areas. My source water has very low kH. The primary reason I never kept Rift Valley Cichlids. Ok, we can buffer pH and add calcium but keeping both stable can pose a problem. As nitrates build the pH naturally wants to drop. This requires constant monitoring and often more than once weekly water changes. Not impossible. Sometimes not practical. Its often best practice to keep fish that your source water can accommodate. Butewe seldom want to. Of course using RO is an option, then adding buffers etc.