The water lettuce has some aphids but I go out and wash them into the pond and the Mollies eat them. I'm really loving these Mollies, they eat algae even string algae, they clean your feet, they eat aphids, they swim into your hand to eat, they are pretty and they stay mostly at the top of the pond. They are great.
I read the news article (linked above) on the pedicure by fish that is offered in salons, they use a type of carp called a Doctor Fish. They are small like the Mollies but they are carp and they love dead skin.