I think what your asking about is adding limestone rock. This would be fine for african lake species as they prefer a higher water alkalinity (8.0+), but for african river species and south american species, this would be no good in the long run (usually like lower alkalinity from ~6.5-7.5).
Basically, the limestone "ocean rock" is made of calcium carbonate which is a slight base. It will raise the alkalinity. Easy way to test for limestone is to put a few drops of vinegar on the rock. If it bubbles, its reacting with the calcium carbonate.