Which Matrix are we talking about? The granulated activated carbon which is excellent ,imo, is called Matrix. They also make a pourous rock like substance called Matrix for nitrate control which doesn't do much beyond providing extra surface area ,ime.
I think aluminum based products for PO4 export can be irritating to some corals if not rinsed well at a minimum.
I've used Cupramine. It's ok but confounding to test for levels compared to straight copper sulfate products.
Their mag ,calcium and alk tests are ok I've used them in the past but prefer Salifert for those tests for ease of use,and easier reading.
On the magnesium;thanks for the detailed information Jonathan, I did not check the labels just the on line descriptions.
I don't use Reef Advantage or the other commercial mag supplements and haven't for several years at least.I use generic mag chloride and mag sulfate ,kalk, baking soda, and calcium chloride in lieu of commercial mixes.
Personally, I won't use the Kent Tech M.even though I've used it in the past without trouble , given the discussions about a mystery impurity and the history of serious metal contamination on another product of theirs(Reef carbon).
Personally, I don't trust the Brightwell marketing in general.
Higher prices on salt mixes don't equate to higher quality ,imo.
Purigen is a nice product; I used it for years along with activated carbon; folks can and do have trouble with regenerating it and need to be careful with that process and avoid drying it out.