Marine aquariums operate at a high pH level from 8.0 - 8.4. These levels will fall in time due to fish food and fish waste.
If pH levels drop too low, your fish may be harmed. Simply dosing Ocean's Blend pH/Alk will assist you to increase pH levels, which contains buffers to prevent high pH swings.
You may also use Ocean's Blend pH/Alk for reef aquariums.