I wonder if our test kits are giving us false high readings. I haven't dosed Mag in maybe 3-4 months because my level were always in bet 1500-1600 and I just thought the my water changes were keeping it up and my tank wasn't utilizing it as much. My recent triton test said my Mag was in the 1300's, I'll double check later. I sent the samples to triton before I switched to AF reef salt. I'll test when I make a new batch of saltwater what the Mag is.
If its still high then I know it's my test kit which is Red Sea. What test kit are you using?