Why you have to change UVC tubes that seem OK

UVC lamps are widely used for sterilisation of well and tank water, and for sterilising surfaces.
Because the wavelength of the UVC light (254nm) is so short, it requires specialist ultra pure glass to pass the required wavelength.

The destructive nature of UVC light means that it damages its own glass tube over time, meaning that while it's still working internally, the actual UVC output may have dropped significantly. It's not something you can see directly as UVC is invisible, but the other wavelengths that give the tubes a ghostly blue glow still shine through the glass.

It is recommended that UVC tubes in continuous use are changed at least once a year.

Never look at a lit UVC tube without eye and skin protection. Even a short exposure will cause serious eye irritation (photokeratitis) in the same way as a welding arc does.
If exposed, you may wake up in the middle of the night feeling like your eyes are full of sand. Although very unpleasant, the eyes will recover over a day or two.

Note that this does not apply to 365nm UVA tubes as used for glowing art type applications or in insect attracting units.