Well, Calcium,Calcium,Calcium!!! Coralline like lots of calcium I have my Calcium at between 380-400ppm(parts-per-million)
I reccomend that you use kent marine turbo calcium to get it to the hight level and then use kalkwasser to keep it there.(any brand is fine).
these aren't too expensive.
Also make sure that you have alot a current on the rock coralline like hight current.