I've got a related question to this one...is the proper way to raise my KH to do it in small increments and test for the KH at each step? My tap water has KH of about 2-2.5, and I'm slightly concerned about the pH dropping if I increase the efficiency of my CO2 injection. I was concerned about the pH dropping too fast, so I started it out on only an airstone. This has dropped my pH from ~7.4 - 6.9, so I'm glad I didn't start out with a more efficient reactor/diffuser. I tried adding about 1/4 teaspoon NaHCO3 to the water (20G tank) but it didn't measurably change the KH or the pH.
How much should I add at one time to a tank this size to start out? I've tried searching and I've seen some conflicting advice. I want to increase the level of CO2 in the water soon (although even what I have now has dramatically increased the plant growth rate in just a few days). I have an SAE and 3 honey gouramis in there, and I don't want to stress them too much too quickly. They definitely showed some minor signs of stress during the initial pH drop.
I'm going to calculate the weight of NaHCO3 necessary, but wanted to check here first. I'm a beginner at planted tanks.
Edit: By my calculations (based on density, molar mass, and the volume of my tank) 1/4 teaspoon baking soda dissolved in 20G should be about 26.5 mg/L. mg/L are equivalent to ppm, correct? So the amount I added should raise the KH by a little over 1 degree. Hmm, maybe it didn't all go into the tank since I dissolved it in a bowl before adding it.