Magnesium gets used up by critters the same as does Ca and Alk (the depletion rate is much slower than that of Ca and Alk but without dosing it will eventually be a downward trend).
700 is kinda low to begin with (although not surprising, many salts seem to come with low Mg unless it's one specifically marketed at having higher levels.). Seawater is something like 1280. I like to keep mine a little bit around 1400 because I think my anemones look better when Mg is there. What I do is add Mg to my waterchange water, I figured out how much to add everytime to make my waterchange water around 1500-1600 and then when I do a water change it bumps Mg up in the tank a little bit (not by much since I can only do about a 8.5% waterchange at a time).
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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