Natural sea water contains between 400 410 mg/L of calcium. A healthy reef environment should have between 380 420 mg/L of calcium. Levels above 420 mg/L are not beneficial. Levels below 380 mg/L should be corrected as soon as possible. This kit is capable of at least 150 calcium determinations and is designed specifically for seawater. The test is based on titration of calcium with EDTA in the presence of a calcium specific indicator dye after the coprecipitation of magnesium and strontium.