<a href=showthread.php?s=&postid=14842216#post14842216 target=_blank>Originally posted</a> by xJake
I just balance calcium, alkalinity, and magnesium using Randy's DIY recipe. Then I replace all of my evaporated water with saturated kalkwasser (lime water, calcium hydroxide, etc.). Kalkwasser adds calcium and alkalinity in balanced proportions, and the levels on my lab's 175-gallon system rarely fluctuate. Kalkwasser also has the added benefit of precipitating out undesirable molecules such as phosphate (PO4), as well as most heavy metals; however, this also can become an issue (in terms of magnesium levels).
The only problem with using kalkwasser so heavily is that you must monitor your magnesium levels quite frequently. Your magnesium level can easily be corrected as well using Randy's DIY magnesium supplement. Magnesium doesn't become an issue on my system until at least a month of regular kalkwasser use (after initial correction). Magnesium (if you didn't know already) plays an important role in how much dissolved calcium carbonate can be maintained in the water at any one time. If your magnesium level is not where it should be (1350-1500ppm), then many times you will run into issues with heavily fluctuating calcium and alkalinity levels (or very low levels).
So, to answer your question, NO, you don't need a calcium reactor to keep SPS. In fact, it can be easier and cheaper without one. Calcium reactors simply provide a bit more insurance that calcium and carbonate (alkalinity) molecules are being constantly maintained within the water column.
It really depends on how your system is set up. I would hazard a guess that many people could run their SPS systems without calcium reactors, but having one just means one less thing to worry about dosing. In cases of extremely high coral biomass and low water volume, then a calcium reactor may be essential; however, most of the time this is not the case.