Yes, pick up an alkalinity and calcium test kit in short order. If you don't have a nitrate test kit, I'd get one of these. Here are the things you should be testing for and where you need to maintain each of the levels:
alk (dkh): 8 - 12
calcium: 400 - 450
magnesium: 1300 roughly...should be maintained at 3x your calcium reading
nitrate: 0 is best, but up to 10 is okay
nitrite: 0
ammonia: 0
phosphate: 0
As a general rule, don't add anything to the tank that you can't test for. I'd highly recommend B-Ionic. I used it for quite some time, but recently switched to the similar product from Bulk Reef Supply because it's cheaper when buying a large amount and my tanks consumption rate has increased quite a bit as my SPS has started to grow out (see pics if you'd like by clicking my username and going to my webpage link).
Also, how are you measuring salinity and where are you maintaining it?