Blanket and pond weed fertilizer

Yes or add it to the compost bin. If you have a pond filter, all the debris that get's collected by that can be used as a top dressing or composted too.

Duck weed wasn't in it as it was pure slime. Anyway, duckweed wouldn't survive in a water butt as there is no light because it's got a plastic lid.

My water butt has a lid which has a 3 inch diameter hole to let the down pipe in. Just enough glimmer of light gets past to keep specks of duckweed alive, although they don't multiply.  I've also got dozens of little black water snails in there. But I like them and they can stay.

When I skimmed my pond I got a bucket full of duckweed. (Small pond - thick layer of weed). I just dumped it in the bottom of the compost bin as a sort of cow pat.   I didn't turn the heap, and when I emptied it 18 months later the duckweed pile was flat but otherwise unchanged. Everything else had turned to compost.

Answer - turn the compost heap over more!