Aquarium light bulbs may not burn out after they’ve lived past their useful shelf lives, but that doesn’t mean they are still good bulbs to use and it certainly doesn’t mean that they need to be run until they burn out. After around 8 – 12 months of use, the T5HO bulbs that hobbyists run 8-10 hours per day reach a point where their spectra have degraded significantly and they no longer emit the same PAR levels. Sure, some folks can extend the life of a bulb, but changing them depends completely on the user and how the bulbs are used long term, so it’s tough to put them on a schedule. On top of that, people cannot really see when a bulbs starts to decline in most cases, so they have to rely on other tell tale signs in order to receive the note. To prevent from ever having to deal with that problem, just go ahead and replace bulbs annually. If you feel that’s too wasteful, give them to a friend who wants to try out different color combos, otherwise they can be listed on the local aquarium forums. Just try to be honest revealing how old the bulbs are.