Hi petmar,
I was considering introducing some Grass carp into my garden pond, with the idea of of controlling blanket weed, that I was getting a lot of at the time.
I sought advice at my local aquatic centre and the guy there seemed doubtfull and said they may eat duck weed in the pond but thats about all.
I think grass may be too tough for them to chomp through and is probably an easier option to remove it by hand if its growing into the water from the pond edge.
I am aware they are used comercially as a weed control and as far as I can see, the grass carp are all sterile to stop them breeding and becoming an invasive alien species, so they may need to be replaced after a few years, as they die off naturally.
Like you, I'd be interested to hear from anyone who's had success with them in a garden pond and moreover are they selective in what they eat and will lily pads and oxygenating plants be eaten too ?