for xenia, i would really consider it one of the most enigmatic corals, and because of its strange track record, i consider it one of the hardest corals to keep.
from n00bs to Pros, some people can keep it in their tanks, and some cannot. Some people have them thriving for years, and then all of a sudden they dissappear. I have seen examples of people attempting to set up 2 or 3 'duplicate' tanks, and the xenia will only survive in one of the three exact same set ups. maybe try another frag in a year, you might have better luck.
for the kalkwasser - you are dripping it right next to the pump for the skimmer, or inside the bubble chamber somehow (which i guess would be tough to do depending on the skimer.)?