I read somewhere that metal halides should be replaced once a year. The problem comes from the light spectrum shifting more towards the red zone, causing the corals not to grow as efficiently and causing algae problems. As a precaution, regardless of what kelvin they are, I will go ahead and replace my metal halides once every year to year and a half. I think I also remember where old bulbs can wear down ballasts and they will start to "cycle". Your bulbs will turn off and on all the time at the end of the bulbs life.