Also, do you have activated carbon in the tank? If not, it should help to remove any residual medication that may exist.Did you swap out everything after the metronidizole? Remove the water too? So it is an entirely new setup and residual medication cannot be the cause?
What salt mix and source of water are you using?
In my tank, green star polyps in poor light (shaded heavily) actually turned white.
Xenia seem hit or miss and can grow like crazy, then all die back. Might be a nutrient or trace element depletion issue, or something else, but no one has clearly identified the reason that I know of.
