Most people raise their KH by adding baking soda to their tanks. I have a KH of 10+ from the tap so I've never had to do this myself. Do a search on raising KH and you'll find the proper dosing recommendations.
You can raise KH by relitively simple (and cheap) means, however you need to know where you are starting before messing with it. You can get a KH test kit (usualy KH and GH are in the same kit) I use therafin kit, and like it. You can use the same test kit for your reef tank too.
Kh buffers PH, by that I mean, with a low KH, 5 PPM of CO2 would lower PH much more than 5 ppm of CO2 would lower the PH in a tank with a high KH.
Thank you all for the answers. Whiskey, are you saying that if you have a higher KH the ph won’t fluctuate as much? Another question, how do you control the co2 bubbles to be at 2-3 bubbles per second?
Ted... With a higher KH it is less likely that you will have extreme fluctuations in PH. With DIY C02 if you use the Hagan ladder (diffusior/bubble counter) you can count the bubbles, you can also control the amount of C02 injected by moving the outlet fitting up or down.
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.
When using baking soda to raise KH I use 1/2 to 1 teaspoon per 10 gallons, which raises the KH by about 2 degrees. Start out with the smaller dose then wait the next day before testing your KH, if still low then repeat intill you get your desired KH. Also I use a small RX bottle to mix the BS with tank water before adding to the tank.