The easiest way to raise pH is by raising Alkalinity. Higher Alk
means better pH stability which is very important. It also is important for invertebrates that build their shells tubes and for corals.
Simplest method to raise Alk
if you have nothing else, add baking soda. It has the bicarbonates that make up most of the alkaline. I would start with 1 lvl tbsp and check a day later to see change. Both pH and Alk
should go up.
Crushed coral will also do the same thing but slower. Better perhaps in the long run, but not as effective for the quick boost.