I use ReefCrest Magnesium Part C from J&L.
https://www.jlaquatics.com/reef-cres...esium-mix.html
They are as good as the BRS Mag or even better as it mix well. I tried the RC bulk Mag, they are tricky to mix without any precipitants. Yeah it is weird, I'm not sure that they use the same components as the Part C. So I just use RC Mag Part C and mix it on a 1G jug, it is more money though.