Tami, a couple of thoughts:
Agreed with spleify, the bio-balls are a bad idea. Since you have a decent amount of LR in the tank, they are unneeded, and probably a source of your high nitrates. And I would second his recommendation that you remove them gradually....remember, nothing good happens fast in reef tanks
Second, I suspect that your Mg test might be off. 750 seems way too low.
I'm not familiar with Seachem's Advantage Calcium, but if it just adds Calcium and not alkalinity, it will get your tank params way out of whack. You might want to take a look at the Ca and Alk articles linked
here by Randy Holmes-Farley, the god of all things reef-chemistry related
One final suggestion...assuming your stocklist is up to date, you should have a very low need for Ca/Alk in your tank. You might want to stop using any additives, and just do regular water changes. That would give you a good baseline for figuring out which additives are useful.
HTH,
-R