The Laguna PT521 is a replacement UV bulb for the following:
The PT521 Replacement 8W Bulb for Pond UV Sterilizer is a replacement UV lamp for the Power Clear UV Sterilizer/Clarifier 1000. This 8-watt pond clarifier lamp provides UV treatment within the inside chamber. This UV lamp sterilizes water to remove harmful microorganisms.
PT521 Replacement 8W Bulb for Pond UV Sterilizer specifications: