The saying “carp eat shit” is greatly exaggerated and has little truth in itself. Carp, like most fish, are predators and feed on a variety of small aquatic organisms, such as plankton, small crustaceans, aquatic insects and their larvae, as well as small fish.
Of course, as with most water-dwelling organisms, their diet may also contain other less desirable components, such as detritus or organic waste. However, eating this type of food is rather haphazard and may be due to the lack of availability of other food sources.
In any case, there is no truth to the saying that carp “eat shit” in the literal sense, and the saying is merely an exaggeration and usually used in the context of criticizing or disparaging carp as a fish species.