It's best, and cheaper, to find a natural buffer. Aragonite works well...how much sand do you have?
Crushed coral works too, but if detritus gets stuck in it, you have nitrates. The solution is to put it in a fine mesh media bag.
You can use ph buffer like Seachem.
What's your alkalinity, general and carbonate hardness. Alkalinity help keep the pH buffered. Needs to be around 10 or 11 KH