The HI758U Marine Calcium Checker HC® uses a 100 to 1 dilution for analysis and thus requires deionized or distilled water absent of calcium to function correctly. Calcium can be found in many freshwater sources and this any amount present in the water used at the C1 phase can cause false high readings. In addition, many hobby grade reverse osmosis deionized water systems may not adequately remove calcium. This can vary based on the types of filters used, their effectiveness and frequency of replacement. Hanna recommends using HI70436M deionized water for proper testing.