If you run your reactor and nothing is consuming alk, cal, and Mg the levels will all increase together at the same rate. That’s the beauty of a calcium reactor is that everything stays in balance.
Boosting your Mg level now may help you get coralline algae growing on everything, which in turn will start consuming alkalinity, calcium and magnesium. If you are looking for a stable environment, it may be in your advantage to do it now as opposed to later, for the simple fact that making a change now will mean that it will be one more parameter that remains the same over the next 5 months

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