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Looking to get this stuff in pharmaceutical grade in large amounts, but most of the search results I've gotten back have been for overseas with minimum orders in the tons.
Local pool supply company but you'll need to have a wholesale account with them. They sell the stuff in 50lb bags and you can get all the pallets you want. It's however commercial grade. Pharmaceutical grade is much more expensive. I mix 50 gallons of BRS equivalent two part at a time in 55 gallon drums. Yes it is risky as its commercial not even food grade much less pharmaceutical grade but I've had no ill effect for over four years. My Mg supplement does come from BRS I just received 8 5 gallon buckets. I can get Mag Flake cheep but I don't need a pallet of Mg thats 40 x 50lb bags. You need to go after food grade materials.
Look you can buy it for half the price at homedepot, has the calcium chloride and the baking soda, Walgreens,dwaneread has the Epsom salt, total coast to make all three parts per gallon I's less then $3 bucks.
Or you can do like everyone else and,pay more for it from bulk reef supply's or if you want the best money can buy would be ESV brand it has the complete package including stuff that you don't get from the home made 2 part.