WordNet records no synonym and no antonym for flatfish. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets
any of several families of fishes having flattened bodies that swim along the sea floor on one side of the body with both eyes on the upper side
Synonyms, antonyms and the broader and narrower relations for flatfish come from Princeton University’s WordNet. Full sources and licences.