That will raise the pH (actually... it'll do it much more so than baking soda. Carbonate is a stronger base than bicarbonate).As from wiki - Potassium carbonate (K2CO3) A known food additive. This adds carbonate without raising pH. The potassium is a bonus if you have a planted aquarium.
I've never tried that myself though. I use an African style additive - baking soda, table salt, epsom salt