If you have a low demand system the best thing you can do is set up an auto-top off with saturated limewater. This will have you set until you start introducing large amounts of stony corals. You can buy Mrs. Wages Pickling Lime at Krogers, mix in two teaspoons per gallon and wallah - ultra-cheap alk/calcium/magnesium supplement. Just be sure that your ATO does not suck up the slurry at the bottom. All of it will not dissolve and will cloud your tank if it gets sucked in.
I top off with limewater 24/7 along with dosing two part due to the demand in my system.
+1 on the three part system. You only really need to add the magnesium once every two weeks to a month, though.
This is all that you really need to dose. In fact it is an absolute must once you get more stony corals and water changes alone will not replenish your alk and calcium levels. I experiment with iodine, strontium, and potassium but I have no conclusive results yet.