Zydia-
I take it you don't want to get the savio UV light that fits right in the skimmer? there is a new model by Savio this year that is supposed to be "better". It sounds like you may need 2 of them for 5000 gallons. One on each side of the skimmer. Someone will be able to help better than I, but i think it goes by the size of the pond in gallons and the size pump you have. If the water flows to quickly, it doesn't have time to kill off cooties before it's pumped on past.