OK, it seems grass carp will not eat crested floating heart (see below).
"Currently, no known insect herbivores specifically feed on this species. Additionally, grass carp (Ctenophryngodon idella), the weed-eating fish used for biological control of many aquatic weeds, have also proven to be ineffective in controlling crested floating heart. The South Carolina Department of Natural Resources has reported winter lake level draw downs of Lake Marion, which expose the plants to drying and freezing, have failed as a means of control. Mechanical harvesting is also reported to be ineffective in the native range of the plant. Because of crested floating heart's fast, aggressive growth and vegetative spread, chemical control is likely to be the best way to control infestations and remove new colonies. Various forms and combinations of herbicides have been used but with no long-term success."
But has anybody seen evidence of ducks feeding on this plant? Seems similar to other lillies that ducks (ringnecks, etc) like to feed on. Since grass carp will not eat, we need to protest all the restocking of carp in future.
Would much rather have hydrilla which ducks love than crested floating heart if they will not eat.