My PH was low when I first started also. I was using RO water with a good quality reef salt, but was told that I should be using salt especially for RO water. Seems to have helped. My PH is now around 8.0 in the morning and 8.2 in the evening.
More likely your live rock simply finished re-curing which stabilized your pH. Let me guess, an LFS convinced you to buy this "special" RO-water salt.