If your going to keep SPS or clams as John mentioned..........2 part will eventually be required to maintain a stable Alk and keep a good calcium level. I'm not sure how much, or what you are testing, however if I can give you a tip I have learned..............test Alk every day, until you can understand what kind of swings your tank will have.
for example...........For the first year, my tank could easily drop over a point in DKH a day. It could be 8.5 one day, and the next day it would be 7.0. Once the Alk get's below 7, it can drop even faster. This is what will kill your coral. I was in the habbit of checking the alk every day for months. I learned that a water change usually brought DOWN my alk, instead of my assumed "correcting" it. This can depend on what salt you use, however Ihave used several brands, and if you let the salt mix for a day, or two like I do. The Alk often drops to 7, and sometimes lower in the new water. If you can't fathom checking it daily for awhile, at least check it right before a water change, and right after, so you can bring it back to what it was.
Once I realized how wildly the Alk could fluctuate, I decided to try a Kalkwasser reactor. This was a bit tricky to understand, and get the hang of, however it stabalized the Alk, and I saw INSTANT results from it. This inspired me to get a doser and I started using 2 part. It is difficult to be consistent when dosing by hand (it's easy to get busy and forget.......often dosing at different times of the day, etc) so I invested in a dosing pump. Totally worth it.
Anyway.....Blah, blah. You get the idea. You do need to check Calcium, and Mag once in awhile......just to have an idea of where it is. Obviously too low will need to be fixed, but too high never seemed to harm anything. Most of what I have read seems to feel that being in the sweet spot with calcium will yield the best results, so I do my best to keep it around 420-450.
That was probably a little over kill. Good luck.