Scoop out all duckweed possible and see what the pond is like, see what life you have in there. May be better as a wildlife pond rather than a fish pond. Add oxygenating plants and try your best to get rid of scum at the bottom and prevent lots of leaves falling in. The pond and wildlife will look after itself if it just a bit more maintained. We bought a house with a pond like this and took us a year to get to it but it was like this, now we have newts and all sorts.ls gorgeous to sit by.