You are not asking the right question.
You need a coral skeleton media -- the Gen-X, Shuran, etc large media are good. Go for the type with the large chunks, it enables a lower pH without turning to mush.
You can add a handful of dolomite (be sure to get the right kind!!!) which will very, very slowly dissolve to keep Magnesium levels over 1250. Dolomite should not exceed about 10% of total media volume.
