Hey Clare, I run a 150W Coralife Clamp on MH fixture and all my corals are doing very well. I have Xenia, Zoos, Leather, Polyps, Elegance, Hammer, Frog Spawn, Clam, Goniporia, Acro, etc. No melting, no deaths, no problems. What ever you LFS is telling you about MH is garbage. I wouldn't anymore money on coral until you get your lighting sorted. In addition a great number of coral require feeding to thrive along with consistant water maintenance. Fix the light, keep your water fresh, and research coral needs before you proceed. Good luck!