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It depends how often you do water changes. If they are very infrequent (1-2 months) then you might need to add Mg.

Otherwise, regular water changes will replace it before it ever becomes depleted.

Mg just needs to be present. It doesn't have to be present in huge quantities. Usually plants do best when there is a 4:1 or 3:1 ratio between calcium and magnesium. So if you keep adding magnesium and the plants aren't using it at the same rate you are adding it then you will get stunting and nutrient blocking problems. Its probably best to just rely on water changes for Mg since groundwater is usually full of both in the correct ratios.
 
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