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I know magnesium sulfate is used in fertilizers... I recall it being on the recipe list for PPS-Pro so i would imagine its fine to use. However, I've been told that if you use tap water in your aquariums and if that source is hard water, then there should be some in your tap as is and therefore, wouldn't need to be added.
If your water is even medium hardness you probably don't need the extra Magnesium. If you're seeing signs of Mg deficiency, by all means dose it, but I wouldn't use it as a preventative measure.