The only way you will be able to increase the KH and lower the pH is by injecting CO2. For example, if you start out with a KH of 4 degrees, your target pH will be around 6.9, giving you an optimum level of CO2 @15ppm, ideal for plants and safe for fish.

The KH can be raised by using baking soda. There is a direct correlation between pH, KH and CO2. When you raise KH, so pH elevates, it's unavoidable, you must use CO2 to lower it!

If CO2 is to low, the plants will eat the KH and convert it to CO2, this process is known as biogenic decalcification, this causes pH to rise and eventually become unstable leading to pH swings.

Best to use a pressurized CO2 system for absolute control.