What types of corals do you have?
If you don't have any stony corals I wouldn't worry about calcium or magnesium yet.
If you only have soft corals so far just focus on water quality. Test your nitrate, phosphate, pH, KH
, temp, and salinity (ideally with a refractometer). Get a good water change schedule going (at least 10%/week) and use high quality salt (I use Tropic Marin) and RO
water to make your saltwater and to top off.
Xenia is a great coral. It will actually improve water quality. It can be sensitive to changes in temp, salinity, etc. If all is well it can grow like a weed, so you may need to keep it under control. Ideally you will be able to frag it as it grows and give the frags to the LFS
for store credit.