Corals Refuse to Live

Hi everyone, my tank has been set up since March therefore its cycled and has been running for 5 months. The fish are doing well however i cannot keep any corals or invertebrates. I've tried a branching hammer, two different mushrooms, a featherduster, two cleaner shrimp, a fire shrimp, many snails and a porcelain crab. All dead. I now have an xenia and colt coral which are on the verge of dying and i want to correct whatever's wrong before they're lost too. After putting the new corals into my tank, they never open up or even come out, they stay tucked away and just die. The only inverts my tank will let live are my hermits.

Tank parameters:

S.G.- 1.025 calcium- 360- 400ppm
ammonia- 0ppm phosphate- 0ppm
nitrite- 0ppm KH- 8
nitrate- >5ppm pH- 8.0 (checked tonight at 8pm)
Temp- 76- 78f

I currently have PC lighting, an MP40wQD and a Hydor Koralia 1950, Remora Aqua C Skimmer, and a sumping filter.

Can anybody shed some light on my issues? I have ruled out copper, i took a water sample to my LFS and they said that no hard metals including copper were present.