To raise your KH naturally, you can add a little calcium carbonate or sodium bicarbonate to your water, making sure it is dissolved first.
What all this means is that it is essential in low-tech planted tanks to measure your GH and KH and ensure that both are kept above 3 degrees or about 50 ppm. A level of 4 to 6 dGH and dKH, or about 70 to 100 ppm, is ideal for both plants and most tropical fish (excluding brackish fish and African cichlids). To raise your KH naturally, you can add a little calcium carbonate or sodium bicarbonate to your water, making sure it is dissolved first. Calcium carbonate is preferred, as calcium is a far more useful supplement for fish. Do also note that adding bicarbonates may affect your pH by making it slightly more alkaline, so monitor this parameter in your tank and in any replacement water to ensure that your pH stays constant for the health of your fish and plants.