Achieving and Maintaining tank stability

[Coralgurl]

Ive been testing my tank every couple of weeks after water changes and am having problems maintaining stability with my parameters in my 180. My 55 is a month since my last water change, tests as follows:
Ph 8.3
Mag 1200
Cal 460
Dkh 6

180-water changed on Saturday
Ph 8.3
Mag 1150
Ca400
Dkh 6

I picked up a doser for the 180 but have not installed it yet. I'm having issues with the corals in this tank, but everything in the 55 is growing like crazy. Both tanks are mostly lps with a couple of softies.

Should I be concerned with these results and be raising them? I'm using salinity salt and salinity is 1.026 in both tanks.

Temp on the 180 is 79 and the 180 is at 80.

Thanks