Here is a good read on light acclimation.
Understanding Coral Light Acclimation
While most hobbyists are very diligent about conventional water temperature and water chemistry acclimation, coral light acclimation remains somewhat unfamiliar. After carefully researching the light requirements of a particular coral, many reef hobbyists are anxious to immediately place new corals in the brightest location, closest to the light source. This may seem appropriate, but in many situations, it can cause more harm than good.
Keep in mind that lighting conditions vary from aquarium to aquarium. No two fluorescent or metal halide systems have identical light properties. Therefore, a coral acclimated to artificial lighting from one system (e.g., a pet store or an online retailer) will need to gradually adjust to different lighting conditions when introduced to a new aquarium system.
Acclimating Your Coral
Regardless of your light setup, place all new corals on the bottom of your aquarium until they begin to adjust. If you employ metal halide lights, place some sort of screening material above your coral during the first few weeks of acclimation. The screen could be egg crate light paneling or fiberglass mesh. Also, reduce the
photoperiod by a quarter for the first week and gradually add an hour each week to reduce the risk of light shock. A good indication your coral has adjusted is when it appears fully expanded and displays full coloration.
Slowly inch the coral to its preferred location over the next month or so. Generally, brighter colored corals should be nearer to the lights. Bright colors signify tissue pigment development that helps protect coral from the UV-light present in shallower depths. Though proper light acclimation is a slow process, it can be the difference between a home reef that simply sustains and one that thrives for years to come.
