KH and magnesium

[Jus_sum_guy]

New to saltwater. Been freshwater all my life.
Loving this new adventure so far!

I just have 2 problems...KH and magnesium levels!

KH constantly stays high and magnesium constantly stays low. What can I do?

Here's some background info...

15g tank that never went through a cycle. Used cured live rock.
Tank has been running for a month
Instant ocean salt with distilled water
A few zoa polyps
Small war coral
Hammer coral
A couple small mushrooms


Salinity 1.026
Calcium 420
Magnesium 1150
Alkalinity 3.42
KH 9.6
PH 8.4

The above levels are always constant. Even after 2 gallon water changes every week
I dose 5ml mg daily to try and get levels up

Any help greatly appreciated!

Thanks