This is from Seachem's site.
Reef Calcium™ is a concentrated (50,000 mg/L) bioavailable polygluconate complexed calcium intended to maintain calcium in the reef aquarium without altering pH.
Polygluconate complexation confers several benefits: it increases the bioavailability of the calcium, it provides a rich source of metabolic energy to help maintain peak coral growth, and it prevents calcium precipitation/alkalinity depletion. Polygluconate contains no nitrogen or phosphorous, thus it is biologically impossible for it to lead to algae growth in a properly maintained reef system.
Reef Calcium™ may be used alone to maintain calcium but will provide enhanced levels of coral growth when used in conjunction with an ionic calcium supplement (Reef Complete™, Reef Advantage Calcium™). Reef Calcium™ is intended to maintain calcium levels; if calcium becomes seriously depleted one should either perform a water change or use an ionic calcium supplement to restore the depleted level of calcium.
Reef Carbonate™ is a concentrated (4,000 meq/L) optimized blend of carbonate and bicarbonate salts designed to restore and maintain alkalinity in the reef aquarium. Calcium and carbonates are essential to all coral growth. If either becomes deficient, coral growth will cease, followed by a rapid decline in coral health. To prevent this you must provide carbonates (Reef Carbonate™) and calcium (Reef Advantage Calcium™ or Reef Complete™).
Used as directed, Reef Carbonate™ will not deplete calcium, magnesium, or strontium which usually tend to precipitate with increasing alkalinity. Reef alkalinity should be maintained at 3–5 meq/L (8–14 dKH). Alkalinity should not be allowed to fall below 2 meq/L.
Reef Complete™ is a concentrated (160,000 mg/L) optimized blend of ionic calcium designed to restore and maintain calcium to levels found in natural seawater without affecting pH. Calcium and carbonates are essential to all coral growth. If either becomes deficient, coral growth will cease, followed by a rapid decline in coral health. To prevent this you must provide calcium (Reef Complete™) and carbonates (Reef Builder™ or Reef Carbonate™).
Reef Complete™ also includes magnesium and strontium in amounts proportionate to typical utilization ratios (100:5:0.1, Ca:Mg:Sr). This allows one to maintain these two important elements while maintaining calcium.