test your change water before adding it to the tank to make sure that's ok.
I would say your magnesium is on the low side, you should aim for between 1250 to 1400ppm, ideally at 1300 to 1350ppm. A low value for magnesium will lower your KH for some complicated reason I don't understand!
I would start by raising you mag to within the range above. Use epsom salts for a one off supplement, you will need approximately 2 grams per gallon of water in your tank to raise it by 50ppm. I would raise it by 50ppm per day until it gets to 1350ppm.
After your mag is correct, retest the KH and then if it is still too low, which it probably will be, look to buffer it. If you try to raise the KH without fixing your mag levels it will probably go straight back down.
Hope this helps