About the 7w PLS Single-Ended UVC Bulb for PX Filtobox 4500 - BOGOF Manufacturer: UVC Bulbs ADD ONE 4W BULB TO YOUR BASKET AND RECEIVE TWO!!! This single-ended 7w bulb is shorter than the norm which means it is a perfect fit for the PondXpert Filtobox 4500 box filter. Note: It is important to change your UV bulb at the start of the season.