Just in case useful I've copied in notes I have on baking soda. Also I should note I'm not against using comercial buffers - I have some liquid calcium carbonate I use sometimes over summer. As long as the product says what is in it and what it will do, I'm happy.
"About ½ a teaspoon will raise the KH by about 2 degrees in 100L of water,so scale this to your tank and off you go. If this method is too temporary theother way is to use a bag of crushed coral in the filter, start with a littleand work your way up, having too much may also raise the pH so take it slow."
I found adding something like a teaspoon to 150gal was enough shift KH around a degree which was all I wanted.
"About ½ a teaspoon will raise the KH by about 2 degrees in 100L of water,so scale this to your tank and off you go. If this method is too temporary theother way is to use a bag of crushed coral in the filter, start with a littleand work your way up, having too much may also raise the pH so take it slow."
I found adding something like a teaspoon to 150gal was enough shift KH around a degree which was all I wanted.