help please: best fish for a pond

Dear all,

I have a pond in NW England about 20ft across and about 4ft deep, with a
regular supply of water running in and out of it. It was dredged this
winter, and at the moment is full of green frothy scum that seems to be
coming up from the bottom in whispy strands that we scrape off regularly.
It is also visited by ducks and the occasional heron.
What I would like is one or more types of fish to graze the pond and keep it
healthy and clear, be compatible with each other and whatever else is in
there, and survive the heron. I know nothing about fish so forgive my
ignorance if this is asking the impossible.
It would also be nice if we could occasionally eat them.

Kind regards, JP