Magnesium chloride actually contains about three times as much chloride as magnesium, so boosting magnesium gives an even larger boost to chloride. Still, the natural chloride level is so high that the effects are small on a percentage basis. A certain mixture of magnesium chloride and magnesium sulfate has no net effect on seawater's major anions (chloride and sulfate).
This was from the reef keeping site...Good stuff.