If you bump your water changes up to once a week you will almost never need to supplement anything, though it is good to keep a bottle of calcium handy just in case, and alk booster or buffer as its called. for emergency purposes. Never add any supplements that you dont have a test kit for. its far to easy to OD on things like Iodine and magnesium, even strontium and molybdenum can become leathal if to high a content is reached, say your following the instructions on the lable and adding 5ml of calcium a day but your only consuming 1ml a day it wont take long for your calcium to be way too high and throw your alk all out of wack.
If calcium levels get too high (over 500ppm), there is a tendency for alkalinity to drop. This affects the buffering capacity of aquarium water and without adequate buffering, pH levels are susceptible to fluctuations. Conversely, if alkalinity gets too high, calcium levels tend to drop as calcium precipitates out of solution. A fine balance must be struck between the various aspects of saltwater to maintain ideal calcium levels.
IMO its just easier to do lots of water changes than wage supplement warfare on my tank.