They need rocks to hide from Heron's though, or those mesh things on top of the water that stop the birds landing close enough to nab the fish.
One of my neighbours is ready to shoot the next heron he sees sitting on his roof. He put a fortune's worth of koi carp into his pond and it scoffed the lot.
Ponds with 'things' around them discourage herons too though. Planters, bushes, that kind of thing. They like open marsh, reed beds and shallow burns. Clear ponds are just like MacDonald's drive in thingies to them.
M