I've got two blue
acro frags under a 250W DE Ushio with a low E glass cover (hood is 5' so glass is in two pieces, pushed together in middle) Directly under where glass meets in middle, I have my two blue acros I raised from tiny frags, along with a new red/green Favia, and below that is my long term green funnel montipora. Soon after doing some work in the hood, I noticed the Favia starting to shrink away from its skeleton on a couple of polyps. Then I noticed the acros starting to brown and fade in an odd pattern, only on one side, the one closest to each other. Then, the montipora, laying lower in the tank, started to turn purple in a band along one side. What the Heck was this. The montipora had an overlap that was shaded and still green, so I knew that light had something to do with its behavior, and figured that the acros and Favia were being overexposed also. I moved the Favia lower in the tank and it recovered but the acros continued to decline, the brown areas beginning to bleach.
Finally, by standing just right in front of the tank, I realized that all teh changes were occurring on a straight line, directly under where the glass pieces joined in the hood, and that I hadn't pushed them together tightly when I'd serviced the hood, leaving a 1/4" gap between, right under the center of the
metal halide bulb. UV burn. I closed the gap, the acros are starting to recover, but the purple remains on the montipora!
I'd heard the warnings about UV covers on double ended lights, I sure believe them now.
