I recommend using kalk at first, unless you want to drop the $$$$ on a CO2 based reactor. It's a fairly simple and complete supplement and can be done very low-tech (i.e. cheap) by just mixing it with your top off water and then toping off the tank regularly. (I prefer to just run my top off water through a kalk-reactor.) That's all you need unless you have very heavy coral population. If you can't keep uo with kalk then you can add some 2 part. I ran the kalk reactor plus 2 part for a year or so and got crazy growth, but that much calcium means frequent pump cleaning! The most complete, low maintenance fix is the CO2 system, but that's not risk free: if you don't have a high quality system with a good monitor it can overdose the tank and kill it very quickly.