The hard corals in my reef tank started growing very well 4 months ago. Chems have been steady, since. About a month ago I noticed growth has slowed, and this week began to notice die back of the base of some corals. What would cause this?

As stated many factors could cause die off. Instead of contacting WWC I recommend going to Reef2Reef to further seek advice on issue. The guru's at WWC will try to help as much as possible but keep in mind they will be limited as they are not directly in front of your tank.

Without getting into a lengthy conversation about your tank specifically its hard to say. There are many many factors. Best to go to your LFS and bring a water sample. Even if you have test kits at home, its always good to double check your kits.

Ron I would contact the store.