One thing that helps a pond stay clean is having half of the pond reserved for vegetation. Now you may have to fence off the vegetation though or it will just end up being snacks
As for fish, larger fish would be fine. Small fish are also considered snacks by ducks.
This isn't exactly what you are looking for but there is some good info here...
http://www.motherearthnews.com/Do-It-Yourself/2002-08-01/Natural-Swimming-Pool.aspx

As for fish, larger fish would be fine. Small fish are also considered snacks by ducks.
This isn't exactly what you are looking for but there is some good info here...
http://www.motherearthnews.com/Do-It-Yourself/2002-08-01/Natural-Swimming-Pool.aspx