When it starts to die, the part that is dying will be a light green or yellow, compared to the very dark green of the live/healthy portion. When you trim it, make sure you do so by pulling it apart. Do not cut it.
Even if nothing is dying, it is still probably worthwhile to pull the Chaeto apart to reduce its density.
As to the saltwater bulbs, I was surprised to see them too. They were in the section with all the other flourescent bulbs. They come in a navy blue cardboard package that says "saltwater aquarium bulb" and make some claim that they are great for corals or something. They are 24" and 18W. They are T10. They don't list the bulb temperature, but they are way brighter than the grow lamps I was using and seem to have made a real difference.