mg/kh problem

[barthsimson]

hi all.
i have problem with mg and kh. i have 55gal tank. my problem is i cant raise the mg and decrease the kh. my mg was 1150 after i have spend a bottle of tailored mg it went to 1200. any little tip help me a lot. thanks.
little info about system
55 gal display tank
20 gal sump
1.023 salinity
ph 8.0
cal 430
kh 16
mg 1200
po4 0
ammonia nitrite 0
reef crystal salt
25 degree