Once the UV lamp's protective sleeve becomes dirty, the pond water and algae won't be exposed to the same level of UV light. So regular removal of dirt and limescale is essential for the optimum operation of your UV unit. The difficulty with cleaning is that the protective sleeve can be very delicate and easily shattered. So buy a spare bulb when you buy the unit.

Limescale build up can be controlled with a magnet, and these may be fitted already, or you could add a limescale/blanket weed magnet to another part of your system.

For cleaning dirt off the sleeve, you'll have to remove it first (and risk breakages), unless you choose particular models fitted with cleaning brushes that you can operate externally. And remember to read the manufacturer's instructions carefully before proceeding.

With time, the effectiveness of the UV bulb with diminish, so replacement becomes necessary.