What kind of corals do you already have?
If you have LPS then you should already be keeping a watch on your calcium and alkalinity. I believe you are supposed to keep your calcium around 350-400 ppm. I believe alkalinity is in a range from 8-12 dkh. Other elements like magnesium can affect corals, but I wouldn't worry about that unless you are running into problems.
Other than that people will tell you low phosphates, high flow, and lots of light.