Ca/mg

Hi im new to this and setting up a reef tank well ben running now 6 months , but im not sure if i have a problem , its around 300 l (65gal) with 100l sump , iv only live rock and two fish , plus pulse coral , my po4 is fine , as is my no2 and ph and density 1.024 but i wanted to try and build up coralline algae , before i added and more corals , i ask in my local fish shop and he gave me kh coralline gro , but im not happy with my ca and mg readings and can anyone give me any answers to get these right , as my ca was 800, and mg 900 many regards and as much help needed