if the metal halide bulb is double ended (DE) - i am assuming it is because you said HQI - then you have to have the glass shield in place. SE bulbs are made of a thicker glass, so they shield the corals from too much UV by themselves, but DE bulbs are made of thinner glass and must be used with a glass shield to prevent UV damage to fish and corals. you do not have to use a glass shield with the PC bulbs.