Provided are automated methods for measuring soluble magnesium concentration in water using fluorescence. The methods employ the use of a pH-buffered liquid and a magnesium coordinating fluorescing reagent. In certain embodiments, the methods may further employ measuring total hardness concentration of the water by displacing any soluble calcium with soluble magnesium and then re-measuring the soluble magnesium concentration. Optionally, the soluble calcium concentration can be determined by subtracting the measured soluble magnesium concentration from the measured total hardness concentration.