To me, water changes are cheap insurance. Even on my 215, I do about 35 gallons once a week. I go thru a bucket of salt about every 3-4 weeks (Reef Crystals, formerly Kent). I DO have a Ca reactor, but it is on a low flow (it hovers around 500ppm). I dose Iodine, Strontium and Coral Stimulator after the water change. I do keep an eye on my Mg, but haven't had a problem with it - probably due to my water changes. My SPS growth has been outstanding. No, I don't worry about the corals being out for a bit - but then it takes only 10 minutes for my water change cycle.