I would like to get rid of it but it would take days to get every last
bit out. Has anyone got any ideas or experience of this?

Duckweed is the world's fastest growing plant. You can read about it he

http://www.mobot.org/jwcross/duckweed/

Some fish do like to eat it, so sometimes biological controls work.

But you might try lots of surface circulation. It needs still water.

C//