Whats wrong with my coral ??

[Aquattro]

If you added a fish and it killed all the other fish, I'd suspect velvet. To clear that up, the remaining fish should be removed and the tank left fallow for 8 weeks.
You'll need to get test kits for Ca and alk at a minimum, they're important values for growing corals. The 20mg/l you mentioned above is either incorrect, or so low it might as well be 0. Ca needs to be about 400 or higher for corals, especially SPS. Which, if they aren't doing well, are dying. You may be able to reverse it, but maybe not.
I'd slow down a bit with the corals until you can manage stability in the tank. 3 months isn't long enough.