Hi Nemo,

You are having single cell algae problems, green
water?

You may have too many fish.
I looked up green tench and golden rudd online and
the tench look like they get large, not sure of the
rudd. Larger fish provide more fish waste for the algae
to feed upon.

But I would
- reduce the number of fish
- clean the pond of any muck, dead plants, if needed
- build a filter that runs the water through plants

I would also consider an UltraViolet filter if you can
afford it. (I don't have one but others really like them)
They won't work on string algae though.

I've heard of barley straw, but also never tried it.

My keys to clear water are low fish stock, low fish
feeding and plant filtering.

Good luck!
kathy