I am into reptiles as well and have kept large lizards and still do. I am impressed at the size and scope of the cage you are building/have built. I think that the amount of filtration will depend on the size of the monitor lizard. If you have an large adult, you will have a large amount of waste or feces. Since the pond is not getting direct sunlight the algea would not grow as fast I think. Therefore, a UV filter may not be necessary. I am new to ponds and don't have much experience so I can't offer much more advice. Good luck!