Im a carbon free bagger....
Used a reactor once, then when changing, threw it in the garbage and went back to bags.
Jeff is installing two reactors in my new sump however. Guess I will have to put something in them....
As for mag, I do measure it now, same as ca and alk. I find dosing a pain to keep a constant level, thus my measurements. Once my levels are set with my reactor, only measure on occasion.