I test for calcium, Alk/hardness and mg. every 2 weeks when I do water changes. If my mg. is a little low I dissolve a couple of heaping tablespoons in a glass of tank water and add it to my sump and then test again and repeat if necessary. All my test kits are Salifert and I replace them every 6 months whether they are used up or not. My 44g cube which has SPS, LPS and softies in it I think I have only had to add mg. once in the last 6 months.
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