swha

Hi, guys
I put a nice piece of elegance coral in my 75 G tank with 220W PC three
weeks ago.
I seemed good for first week, but it doen't pop up well after that.
One reason I thought is insufficient light. However, although I placed it
right under the pc with sand bed, it wasn't getting well.
The other reason I think is that it suffered from stress due to large
portion of water change (30%).
Current water parameters are:
calcium level - about 400 ppm.
Nitrate level - around 5 ppm
Ammonia & Nitrite - 0 ppm
PH - 8.1

It's getting worser as time goes on.
Do you guys have any idea to make it health again?

Thanks for your help.
Sungwon