A clear plexi lid will not interfere with light transmission, so long as it is kept clean (salt spray will accumulate on the underside of the lid. The problem you may run into is with heat. The lid will hinder evaporation which is one of the best methods of heat transfer in a reef tank. With the lid, the heat could build up more in the tank, depending on your lights. What size and depth tank, and what tyoe and wattage of lights are you running? The heat transfer issue could be resolved with a fan on your sump, if you are running one. What type of filter are you running?