ok, so I need to keep my Ca and Mag at 420+ and my mag 1400 and dk at 8-9. how do you get your dk down, mine is at 11dk now. also what is a good test for Mag?
Why do you need to get it down? All of those levels you posted including your alk are just fine. If you do want to get it down, you can let it come down naturally, through water changes, or if need be 1 teaspoon of vinegar.
Your system will take down the ALK. If it doesn't take it down to your liking, you can knock it down in your mixing container by dosing Muriatic Acid.
I use Salifert for Mag test.