Yamitsu Spares - UV 'O' Ring Seals these 'O' Ring Seals go over the outside of the quartz sleeve and should be replaced when adding a new quartz sleeve.

These seals become brittle with time and need to be replaced at least once a year to avoid water bypassing the seal and coming in to contact with the electric bulb.

Note:
Dont forget you need to order two for each U.V