couple problems first the reflectors for used for many residential/comercial MH fixtures isnt really good for aquriums, they are designed to provide a specific beam angle, not to aid in spectral intesity (increasing PAR, enengy of the light used for photosynthesis) , Second Metal Halide represents a large amount of different gas lamps, dpending on the gas and inert compounds in the bulb you may not get the right colortemp or PAR values from residential/comercial lamps.