Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
carbonates keep the pH stable. When you add CO2 that drops the pH.
Most fish and plants are just fine with the shift that CO2 creates.
Nitrifying bacteria use the carbonates as a source of carbon.
These bacteria do not grow well when the pH is less than 6.5.
While the tank is a planted tank, and plants are the current bio filter, there ought to be a decent bacteria population, too.
I would add whatever you want to raise the carbonates to at least 2-3 German degrees (2-3 drops in the API test) Baking soda is about the cheapest, and dissolves quickly. Some people worry about the sodium, though. Calcium carbonate will work, too. It adds calcium (obviously) as well as carbonate, and does not dissolve so easily. There are other materials that will add carbonates. Potassium bicarbonate, for example.
You could add things like coral sand, limestone sand or oyster shell grit to the filter (I use nylon stockings for bags). In a tank where I wanted a greater effect I used coral sand blended with the other substrate material.