Carbonate hardness (KH) is important for nitrifying bacteria. For every 10 ppm of ammonia converted to nitrate very roughly 71.4 ppm (71.4 KH or 4 dKH) of carbonate is used up. Each gram of most ammonium salts requires very roughly two grams of sodium carbonate or three grams of sodium bicarbonate (baking soda), A reference is useful: “How Alkalinity Affects Nitrification”, Barillo, 2015
“During nitrification, 7.14 mg of alkalinity as CaCO3 is destroyed for every milligram of ammonium ions oxidized. Lack of carbonate alkalinity will stop nitrification. In addition, nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.8. Therefore, it is important to maintain an adequate alkalinity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers.”
Note this industry standard of 7.14. assumes the equation of:
4NH3 + 4CaCO3 + 7O2 + 4CO2 —> 4Ca(HCO3)2 + 4HNO3
It has to do with the species of carbonate. At a pH of 6.2 to 8.3 virtually all the carbonate in solution is actually bicarbonate per a complex graph called a “Bjerrum Plot”.
So driving calcium carbonate to bicarbonate is a one to one equation. And it looks only at the nitrogen in the ammonia as that is technically how test kits look at it. So the weight ratio become 100/14 (100 is the molecular weight of calcium bicarbonate and 14 is the molecular weight of nitrogen) or 7.14.
There is a lot of research which says carbonate (typically either sodium carbonate or ground limestone which is calcium carbonate) needs to be constantly added to aquaculture systems in order to have good nitrification by bacteria. So when one is cycling one needs to constantly add sodium or calcium carbonate powder unless one has high KH water to begin with.
I have used baking soda (sodium bicarbonate) to raise the pH for many years. It turns out this is supposedly not a good course. It is supposedly better to add sodium carbonate or powdered calcium carbonate (“agricultural lime”, not “slaked lime”). Carbonate is supposedly better than bicarbonate for keeping pH neutral (although I’ve never seen a problem). Note that in theory sodium carbonate will be the best salt (pure sodium carbonate can be obtained over the internet as “washing soda” or “soda ash”). Potassium carbonate will also work well.
It is also very beneficial to add crushed coral to the filter. When cycling I monitor the pH and keep it above 7.4 with baking soda. It is sometimes surprising how much baking soda is needed to do that.
Note that this position is a change as of 08/07/2022. I had previously had a severe case of confirmational bias which said KH was unimportant during cycling, even though I was adding tons of baking soda every time I cycled a tank. Confirmational bias is real!