Saw the seachem on Amazon. Think I’ll buy that.Seachem Reef Advantage Magnesium and Kent Tech M Magnesium seem to be the most potent for the price. I've seen other brands that cost more and are less potent per mL. LiveAquaria has Magnesium Chloride and Magnesium Sulfate in bulk for relatively inexpensive so you can mix your own DIY Randy Recipe. BRS.and SaltwaterAquarium.com probably have similar for similar prices.
Recommendations for calcium?