will definately be a concern but should not cause what your seeing. Still a good idea to get that back up to NSW
levels. Preferabley closer to 410-415 ppm
given your SG
. Low Ca
will cause poor growth and a low density skelaton.
The two main problems I see that could definately be at issue is the light acclimation and tank age. How was the new tank set up?
As far as the light is concerned, altering the photoperiod doesn't do much at all when upgrading to MH
from a fluorescent fixture. They will burn just as easily in an hour. Increasing the distance from the light source helps but is not nearly as effective as shading the lights by using several layers of screen door material. This reduces the intensity and allows the corals to gradually acclimate to the new light. Every week or so you can remove one layer and gradually step up the intensity over time. Something I would suggest you still do.