What the #$%^ is going on every time i do a water change.

[zum14]

So I got my mag test this morning, tested using the refrence fluid, tested properly, then i tested my tank, 1125ppm, tested the salt mix i have, 1125ppm. Retested the reference and it was exactly as it said at 1300ppm. So there doesnt seem to be a swing between mix and tank to cause the commotion but it is low is it not? How would someone go about getting that up to the proper levels without just adding a bunch of mag all at once and shocking the system? Periodic small doses? Mag heavy water change? Mag heavy fresh top up? Thanks.