Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
White paint, preferably flat, reflects the most light for any kind of reflector with a non-optimized shape. Just ask any grower of a certain illicit plant. Though I'm not one, there's plenty of knowledge that carries over to our hobby.
That said, sometimes it's beneficial to use aluminum foil or tape. Heat rises, and even in a plastic hood with plenty of ventilation holes, it may eventually melt or warp the plastic directly over bulbs. I know I'm not the only one who's melted an incandescent hood by putting in CFL's, then removing the aluminum reflectors to make a little extra room for the larger bulbs. Those reflectors aren't just reflectors, they also keep heat from rising straight to the plastic, and spread it out like a heat sink.