Randy Holmes-Farley Part 3A (preferred two-part magnesium)
magnesium
MgCl2·6H2O + MgSO4·7H2O
Manufacturer data
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What one dose does
PARAMETER
1 ml / 100 L
Magnesium
+0.4702 ppm
To raise magnesium by 50 ppm in 100 L: 106 ml.
How it’s used
5 cups MgCl2·6H2O + 3 cups Epsom to 1 US gal -> ~47,000 ppm Mg, ~70,000 ppm SO4, ~86,000 ppm Cl (published; no gram weights given). Implied weights ~934 g MgCl2·6H2O + ~680 g Epsom (stoich check: 47,224 mg/L Mg, 70,052 SO4, 86,051 Cl). Add 610 ml each time one gallon of Part 1 AND one gallon of Part 2 have been used (Recipe #1); 305 ml for Recipe #2.
Cautions
• Part 3 cannot be premixed with the calcium or alkalinity parts (sulfate + Ca -> CaSO4; Mg + carbonate -> precipitate). Some CaSO4 haze in the jug is expected; shake before dosing.
• Ratio choice: sulfate-rich blends (RHF Part 3A, 5:3 by cups = ~1.4:1 by weight; BRS 2-part version ~2.8:1 by weight) are meant to be used WITH a CaCl2-based two-part, whose chloride load needs extra sulfate; a stand-alone Mg supplement for corrections is better at ~10:1 MgCl2:MgSO4 by weight (RHF, relayed; BRS general mix 1285 g:124 g).
• Raise Mg no more than ~100 ppm/day; wait 5 days off-dosing before setting a maintenance dose (BRS).
• Cup measures are approximate; weigh where possible.