theres no such thing as a uv sterilizer for a pond, for pond use they are known as uv clarifiers and are mainly used to kill single cell algae that can turn water green or brown.
as for the original question, uv sterilizers are a fine addition to tank filtraton although it is not at all a requirement for a well balanced freshwater aquarium that is cleaned regularly and isnt over stocked. The discus tank on my avatar has a uv sterilizer on it but it was left over from a reef tank.
The main thing you need to know about a uv sterilizer is that once it is on a tank it must remain there for the life of the fish since it weakens their natural immunity to diseases as a result of the sterilized water. If you use a uv sterlizer and take it off after a few months chances are you will lose all your fish.
a better choice for you would be a larger xp in addition to the one you have. gl