Arcadia Ultra Clear 24w U.V. Tube Compact PL 4 Pin

Arcadia 24w U.V. Light Tube PLL Four Pin Fitting 

An 24w U.V. Light tube PLL Four Pin Fitting, 315mm in length end to end 

These are quality replacement U.V. light tubes.

You should replace the UV lamp ideally every 6 months use, even if it is still glowing as its UV output will be greatly reduced and after 1 year the will be ineffective against killing algae in your pond.