These UV replacement bulbs are designed to fit the following UV sterilizers;

3w = Aquarium Fish Tank UV Sterilizer 3w with Powerhead option CUV-303
5w = Aquarium Fish Tank UV Sterilizer 5w with Powerhead option CUV-305
5w = 500L/H Aquarium Internal Filter with 5W UV
10w = Aquarium Fish Tank UV Sterilizer 10w with Powerhead option CUV-310