If your tap water has a pH much below 7 I wonder if you have copper plumbing. Low pH water can erode copper plumbing pretty fast.
Coral is calcium carbonate, which dissolves in water, but slowly. When you add it to the filter it adds calcium carbonate to the water, raising both the GH and KH. But, you have no control over how high it goes. If you do weekly 50% water changes, you can limit the increase in KH and GH to twice what it is in one week, so that might be a good way to use coral.
You only need a KH of 1 to 2 dKH to buffer the water against unstable pH due to CO2, so there aren't many good reasons to raise it any more.
Have you checked your water company to verify that they deliver low pH water?