All great suggestions, but a canopy is the best way imo. Eggcrate and mesh will keep most fish in, but I've had plenty of smaller Wrasse and Anthias get through those small holes over the years. It does just as it's description suggests also. It diffuses light. I'll bet at least 30% or more light gets bounced off the egg crate and out to the room. FWIW, I can notice a huge difference in how lit the room is when I remove the egg crate from the one tank that still has it when the lights are on.