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Originally Posted by Randy Holmes-Farley
A certain mixture of magnesium chloride and magnesium sulfate has no net effect on seawater's major anions (chloride and sulfate). All that is necessary for such a recipe is to add these two ingredients in such a ratio that they add chloride and sulfate in the ratio naturally present in seawater (which is 7.1 to 1 on a weight basis and 9.6 to 1 on a per ion basis).
To perfect such a recipe, it's imperative to know the amounts of sulfate in Epsom salts (39%), the amount of chloride in magnesium chloride hexahydrate (34.9%), and their bulk densities, because most aquarists will use a volume based measurement (1.05 g/cm3 for Epsom salts and 0.85 g/cm3 for magnesium chloride hexahydrate solids). Taking all these factors into account, the desired volume ratio is 10:1, MAG flake to Epsom salts, as a supplement; for instance, 10 cups MAG flake and 1 cup Epsom salts.
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http://reefkeeping.com/issues/2006-07/rhf/index.php
RHF wrote this article when calcium reactors were THE thing. 2-part dosing is more common now though, and we add a lot more chloride ions through calcium choride, so a better version for 2-part
particularly if you are dosing calcium chloride heavily is 5:3 chloride to sulfate to make up for all the chloride we're adding via 2-part.