Check out this review on our HI758 Marine Calcium Checker for Reefs.com https://reefs.com/2018/02/05/hanna-instruments-hc-marine-calcium-checker-definitive-review/ The HI758 Marine Calcium Checker HC® 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 C1 phase can cause false high readings. In addition, many hobby grade reverse osmosis deionized water systems may not adequately remove calcium. We recommend using deionized water with low conductivity or pure vapor distilled water commonly found in most pharmacies. Never use tap water to clean your cuvettes. This is especially important for the HI758 Marine Calcium Checker as calcium is commonly present in tap water and residual amounts present in the cuvette will elevate results produced by the Checker.