You need to bring your carbonate alkalinity up.
Try adding ~6 tablespoons of baking soda.
If you are using a calcium reactor and your calcium levels are very high, you may see some precipitate when you add the baking soda.
If you're interested in maintaining stony corals, baking soda will probably become your first-line supplement for alkalinity. Corals need both calcium and carbonate to build their skeletons. You need to balance these adequately in order to maintain sps corals.
The water you add in each water change will probably at pH=7, and the water you remove will contain some of your alkalinity, which is buffering your pH at 8.33. It's fine if you want to do large, frequent water changes, but remember that you will have to replenish your alkalinity with small amounts of bicarbonate (baking soda).
There's a great article online about how to balance calcium and alkalinity that I think everyone should read if they plan to maintain stony corals, especially delicate ones.
http://www.advancedaquarist.com/issues/nov2002/chem.htm
Hope this helps.