A concentrated (10,000 mg/L) gluconate strontium supplement for the reef aquarium that will restore and maintain strontium to levels found in natural sea water. It is most useful in densely populated aquariums requiring a continuous supply of strontium. It has a pH of 8.2 and is neither acidic nor caustic. Gluconate complexation confers several benefits: it increases the bioavailability of the strontium, it provides a rich source of metabolic energy to help maintain peak coral growth, and it inhibits strontium precipitation/alkalinity depletion. Gluconate contains no nitrogen or phosphorous, thus it is biologically impossible for it to lead to algae growth in a properly maintained reef system. 500 mL treats 8,000 L (2,000 gallons).