Trouble Raising Mg

[spawn]

Hi everyone,

I'm having trouble raising my MG
temp 76-78
Nitrate 0
PH 8.4
CA 420
KH 9
The only test kit I don't have is for phosphate. I've been dosing with Reef advantage Mg. for almost 2 weeks. At the start the Mg was @ 1050, now it's only up to 1100. I've done 1 water change today, but have almost added a 300 gram bottle in those 10-12 days. At recommended maximum doses. Does anyone know why I might bee having trouble raising the Mg. Any help appreciated. thanks. oh yeah 65 gal setup