Yes, Mag is a very important, often neglected component of good sea water. Probably because the tests kits are expensive, and people take a pass, since coral mag uptake is small compared to Ca and CO3. its several times the concentration of calcium and other things. It is a small part of coral structure, but very important to keep high levels of Ca and carbonates in solution without precipitating . Dont ask me to explain why. Just go to RC chemistry forum and read some Randy Farley chemistry posts.
yes, most things have heavy metals in trace amounts. including the snow melts i use for the DIY 3 part. but you know what, a little piece of poly pad in the sump filter somewhere with full flow, will remove every bit of it. or some of the other heavy metal absorber resins and pillows. not a big problem. I always keep a 2 x 4 inch chunk of poly pad in the filter somewhere, and after a few months when it turns dark brown, replace it.
Yes, aragamite is finely ground fossilized aragonite and ancient coral reef rock. Perhaps from the mountains of Idaho? but it is no more soluable in 8.2 PH salt water than a snail shell, clam shell or shrimp shell . it mostly settles to the sand bed and makes mud which can eventually go anerobic. It does disolve in water below PH 8. I added 6-8 heaping tablespoons of the stuff to my 20 gallon RO makeup water trash can over a year ago. i use and add new RO everyday mixing it up milkiy white. and its mostly all there on the bottom. a very small amount is disolved by the PH 7 RO water until it raises the PHto 7.8 -8 then stops disolving. it adds about 1 DKH of alkalinity and very little calcium. less than straight tap water. remember, our aragonite sugar sized sand comes from the shallow reef beds. why didn’t it just all disolve away in the ocean?