Your tank looks great, Tasso! Fish are happy and healthy.
I followed the instructions per the BRS video, 5 cups Magnesium Chloride and 3 level cups of Magnesium Sulfate to a one gallon jug, fill with RO/DI water and mix.
the BRS reef calculator says I need 725.8 ml of Mag solution to go from 1260ppm to 1320ppm. That seems a little high to me.
Unlike alk and cal, you can add a lot of mag at a time and still safe for the tank. The recommendation is no more than 100ppm per day. So your 725ml is only equivalent to 60ppm. Just spread that throughout the day and you will be fine.
This is a video of my GFO reactor. Your don't need a lot of flow thru it. As you can see there is only a small area that the GFO actually tumble.
http://s667.beta.photobucket.com/user/SimonSKL/media/GFOReactor_zpsc65c0554.mp4.html
I have my GFO reactor first before carbon after the return pump. My Blowhole pump runs on high all the time and I feed 4 outlets with the outflow before going to the DT: GFO, Carbon, UV, and calcium reactor.