If you check out this article, it explains how your sulfate levels increase by continuing to use epsom salt to the aquarium. Unless you change water. The magnesium is always being used by plants corals etc, but the sulfates stay behind.
Are Sulfates good?
Do-It-Yourself Magnesium Supplements for the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com Chemistry and the Aquarium
If we use magnesium oxide, would we only be left with oxygen?