What is the current pH?
What are you trying to get it to?
Crushed coral in the filter is one of the best methods, more effective than as a substrate because of the flow. If it still needs to go up it may be worth it to just get a proper buffer, one that will bring in to a specific pH, not just up or down. A powdered buffer will be easy and economical and you can figure out how much to add each eater change to keep it correct.