The simple guide to supplementation is: test first, supplement second.
Water changes will take care of most everything you need. That doesn't mean you don't need to add anything. Test your water, then add what your water needs. I add a dose of Seachem's Reef Buffer once or twice a week, but only when I need it to keep my KH
around 10. I usually end up adding calcium every two weeks, but again, only after I test for it first.