|You are right; the slow reef growth is related to your pH levels. The calcification of hard corals and marine algae occurs more efficiently at a higher pH, and for a reef aquarium like yours, it should be your goal to maintain pH levels in the 8.3 to 8.5 range.
The pH levels in a tank are directly related to the balance of various trace elements in the water: carbonates, bicarbonates, magnesium, borate, and potassium, to name a few. Since the biological processes in filtration and the corals deplete these elements at a faster rate, they need to be replenished periodically to maintain a high and stable pH.
To do this, a "pH buffer" is recommended. There are a number of buffers available on the market designed for various applications. In fish-only aquariums (without a large number of corals), the depletion rate of trace elements is generally slower, though the rates vary more widely from element to element. In this case, a general marine buffer would be appropriate, such as Reef Pure Marine pH Buffer, SeaBuffer, Marine Aquarium pH Buffer, and Marine Buffer.
In a heavily stocked reef aquarium like yours, many of the elements such as magnesium, are used up much faster and need to be replenished more frequently. There are a few buffers which include the extra elements necessary to correct your aquarium pH. Some of these products include Reef Pure Marine KH Buffer w/Magnesium, Buff, Reef Buffer, and Reef Builder.
It is important to monitor the carbonate hardness, pH, and calcium levels regularly when using any of these products.