Ca/Alk/Mag Mess . . . How to fix?

[Enigma]

So, I've gotten myself into a terrible mess with Ca/Alk/Mag.

I started dosing Reef Builder a while ago to bring up my Alk. I had brought my Mag up to high levels in an attempt to battle an algae, but that killed four Mexican Turbo snails so I let the mag fall naturally. My calcium had always tested high: around 520 or so.

As it turns out, my calcium test was completely out to lunch. My new test (which has been verified against a known sample) is telling a very different story.

As it stands right now:
Alk: 8
Ca: 300
Mag: 1250

I bring up my ca, and my Alk drops. I bring up my Alk, and my ca drops. I did just dose magnesium. I have no idea what will happen as a result of that.

How, exactly, do I fix this? I have calcium, magnesium, and reef builder. Should I even bother trying to fix this with three separate products, or is there something else I should be using?

Edit: I have read Myka's guide . . . I'm just feeling like I need some specific direction.