im sure your lfs guy knows his stuff, but i think he confused you a bit. Ph and alkalinity are different. alkalinity is more or less the waters ability to resist change in your ph. but to make things simple, let me say that to keep your corals happy and growing, you need a ph test kit, you will need alkalinity test kit, a calcium test kit, and some form of a buffer and calcium additive to add to your tank. Let me give you some numbers to shoot for, although this isnt gospel, just a figure to start with.
keep your ph at 8.2 to 8.4
keep your alkalinity levels around 3.5 to 5 or 7-12 dkh
and i would recommend keeping calcium levels 450 or so
Once again, those numbers arent a must or gospel, just a good range to aim for. if after testing your water and your ranges arent in that vicinity, remember to make adjustments gradually. hope that helped a little, and im sure there are people on here to help you out much more than i.