Getting Seahorses Thursday Water Param Help Please

Hi Everyone, so I have a 46 gallon bow front with a few snails and shrimp in it, and I'm getting seahorses Thursday I'm hoping you can get me some help so I don't accidentally hurt the poor guys from low water quality. Here's my current parameters
Tank Age: 6 months
Copepods and ampipods: quite a few and large
Last Water Change: 3 months
Lighting LED: white light 12 hours a day
Filter: hang of back
Air stones: not currently used but available
Tank: 46 gallon bowfront open top
Current Inhibatants: 2 cleaner shrimp 6 snails 10 hermits, 1 gsp coral, 1 short coral
-The gsp is turning white and the other coral shriveled up from me adding reef buffer

Ammonia: 0
Nitrite: 0
Nitrate: 0
Ph: 7.8
Calcium: 560
Magnesium: 1670
Salinity: 1.026
temperature: 76 degrees f
KH: 200ppm
GH: > 100 drops/ 2000ppm!? brand new kit but could be bad solution?

I frequently use reef buffer to raise the ph, i'm not sure if this is doing damage?

Some things i have available to use are: ph up, ph down, water softner pillow, water for a water change, baking soda, magnesium, calcium, reef mix salt.

What changes should i make?

Also is that GH actually possible? and any idea what happened to the poor coral or how to save them?