Anything bigger than a pound, a Heron would have problems with. They will take fish a bit bigger but have trouble necking them, they tend to pick at them from the gills inward.
For a heron to spear a fish in the side, it must have been on it's side in the first place. Plus a Heron only weighs between 2 and 4lb. So the 20lb Carp thing is a bit of a stretch, unless the fish was sick.
Also a Heron can only fish as deep as it can wade.
I have resident Herons here and the pose no problem to me and I like seeing them.