Iam not dosing I have never done that it has always been that high I think the lowest it's Been is 1580 the other week
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What's your salinity as that can affect the level of Magnesium quiet a lot ?
Also have you checked how much Mg is in the salt mixture your using when mixed to x salinity?
The Mg test kits are very sensitive and it only takes minor errors when doing a test and the results can be way off. I use the Red Sea test kit and again it's very sensitive.
In my tank the Mg level has always been around 1380-1440ppm when using the Red Sea Coral Pro salt which at 35ppm salinity mixes to Mg of 1350-1430ppm. Your salinity doesn't need to be out much and it will affect the readings quiet a lot.
If it is at 1580-1600 and as long as everything is healthy, I would just deal with it as part of water changes but just check what Mg is actually in your salt mix because that's what your putting back in.
Also if you get any 'odd' test result wether Mg or others, double check before reacting. When I first started my Mg was up and down like a yoyo. But it was test error, which I learned after dumping loads of Mg powder in the tank! To no I'll effect as it turned out.