If the pH is high in a fresh water aquarium then just place crushed coral in mesh bag and place the bag under gravel substrate.About 1kg of coral need per 40litres of water to buffer the water to hold a pH around 7.6.
Purchase chemicals in tablet, liquid or powdered form to keep the water at the proper pH. Follow directions on the package. this is much easier than constant testing and adjustments.
Check your local pet store for the many products available to either increase or decrease the pH. You may want to start with a complete water change. Be sure to add some fresh new water each day and continue checking the pH levels daily.For best results,be sure all chemicals you are using are the same brand.