Uv filter

Hi, I’m just trying to work out a set up for a pond that I’ve been left with(around 8/9000L with one large koi, 3/4 very small koi and 5/6 goldfish). Currently, there is an old UV clarifier(I can’t see any glass for a light to indicate that it works), a separate pump(10000L/H) and a DIY filter with sponges and balls which is higher than the pond creating a “waterfall”. I am firstly looking to purchase a uv filter, the water is very gr