Pinda, are you talking about corals, that you buy or do you mean coraline algae (the pink/green/purple) stuff that grows on the rock/glass or other equipment?
If you are talking about corlas, then I agree with DT.
If you are talking about coraline, then lighting/salinity and I believe Ca will help it to grow. You will need a piece of LR that already contains the algae, and it can/will spread.