"Kate Brown" wrote
We investigated Grass Carp, but they eat other kinds of weed, I think, but
we were told, not lemna minor. It's a very small pond, and Koi are rather
large, aren't they?

Grass carp get very big too.
Koi do get very large if given the space.
Here are some small ones for sale...
http://www.nishikigoi.co.uk/index.ph...1&home_style=1
or if you want the most prized type...
http://www.nishikigoi.co.uk/index.ph...=1--RegardsBob Hobden