Crushed coral, is largely made of Calcium Carbonate, (CaCO3). It is used to buffer water at high pH levels. Adding CaCO3 will never lower pH, as it slowly dissolves it will raise pH.
Coral or any other material that fizzes if you put some vinegar on it should not be added to freshwater tanks, the exception being rift valley tanks where it is often used to keep the pH high.
RO is the way to go. Chemical buffers which reduce pH are usually based on phosphates, these are used enthusiastically by algae, and can cause all kinds of problems.