Replacement metal halide bulbs.
Remember, when putting in new Metal halide bulbs you should never touch the unit with your hands. The reason you do not want to do this is the oils on your hands will come in contact with the glass lamp thus shorting out the life cycle of the light.
You will be amazed with the amount of light in your aquarium you see with these HQI and mogul base units.
Bright can't even describe them.
Remember the blues fade after 4-6 months so the bulbs should be replaced in metal halide fixtures including HQI. If not your corals my die as they depend on this blue spectrum to photosynthesize.
One reason why customers love MH is due to the shimmering effect that the blues can cast over the water of their tank. See video below.