Yes, usually it would just be one or the other. But depending on your tank size and what you have it stocked with, at some point your dosing requirements may get so high, that you will be using lots of two part, and that can get expensive. Where as the cost of running a calcium reactor can be considerably cheaper over the long run. I have a lightly stocked 120 gal and have been using 2 part for a year or so now with fantastic results! I have myself been asking around and am contemplating introducing a calcium reactor into my system, although I still think I will start off with a kalk reactor first (even cheaper and easier than the calcium reactor) and give that a go. Let me know how it goes, and have a great one!