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To raise calcium by 20 ppm in 100 L: 54.1 ml.
Ca-equivalent mass: 500 g x 0.2726 / 0.3611 = 377 g pure anhydrous per US gal (~397 g at 95% purity) gives the same ~37,000 ppm Ca stock as Recipe #1.
• Dissolution is strongly exothermic - add slowly to cold RO/DI in a heat-safe container; never add dry to the aquarium.
• Hygroscopic: absorbs water from air, so actual Ca per gram drifts down after opening; store sealed.
• Randy: use about 20% less by VOLUME than Dowflake; by MASS the Ca-equivalent of 500 g dihydrate is ~378 g pure anhydrous.
• Ice-melt pellets often contain other salts/additives; use only if 100% calcium chloride.
377.5 g made up to 1 US gal total.
reefkeeping.com/issues/2006-02/rhf/index.php