Generally, you want to bend the metal (spectral aluminum is probably best for our purposes) into the shape of a parabola. The light bulb should be placed at the "focus" (high school algebra - you finally find out there was something useful in that class!). With a parabolic-shaped reflector, you can end up putting more light into your tank from the reflector than directly from the bulb itself! (the reason is, that with a properly-shaped reflector the light will be hitting the water surface close to perpendicular to the surface. The closer the light is to perpendicular to the surface, the more of it that actually penetrates the water surface. Light directly from the bulb is only close to perpendicular right under the bulb. The further to the side you go, the lower the angle and the lesss light that penetrates.)
