If you ask, “When should I replace my metal halide bulbs?” you will likely hear it authoritatively stated “every 12 months.” With high Kelvin bulbs, it may even be recommended that they be replaced as often as every six months. These recommendations, however, have very little research to back them up. While a considerable volume of research is available on bulbs' spectra and output thanks to dedicated aquarists such as Dr. Sanjay Joshi, et al2-19 and Joe Burger1, very little research has been done on the useful life of metal halide bulbs. Joshi and Morgan4 did examine the spectra and light output of some used 250-watt MH bulbs. This study included five different models of used bulbs. None of these used bulbs were double-ended. While this study provides some very useful data, there are some limitations to the study. The bulbs came from a variety of different sources and the authors admit that “often, the exact number of hours the lamp was used was not known.” The age was “given by aquarists in the total duration of time the lamp was in use.” They go on to note that “the operating environment for the lamps may be very different, leading to variation in the lamp output arising from the variation in operating conditions.” This introduces considerable uncertainty to the measurements. For example, one of the bulbs tested was a 10,000K Coralife bulb. They had three used bulbs that were described by their users as “one year old.” The PPFD values for these bulbs varied, however, by over 50%. It is impossible to know whether this variability was due to intrinsic differences between the lamps or to differences in time and the environment of use. They do conclude for two of the other tested lamps, though, that “these lamps could be used longer than a year” based on the limited drop in light output seen in their study. They also observe for the 20,000K lamp that “these lamps should probably be changed earlier than the lower CCT lamps” due to a greater loss of output. The authors published an additional article3 with similar evaluation of 400-watt MH bulbs. While these studies are quite useful, I do not know of any prior study of 250-watt MH aquarium bulbs prospectively over time utilizing multiple “identical” bulbs under “identical” conditions for a known duration of use.
Since I acquired a quantum meter for measuring PAR, I decided to document the output measurements of my metal halide bulbs beginning from the time I put them into service, in order to see how their output declines over time under similar operating conditions. I am currently using 250-watt Phoenix 14,000K double-ended (DE) bulbs, and I am unaware of any study on these bulbs after extended use.