Order Pleuronectiformes: 

Adults are not bilaterally symmetrical, juveniles are with one eye migrating to the other side of the cranium; dorsal and anal fins have long bases Bodies are  highly compressed, somewhat rounded on eyed side and flat on blind side with eyes that can protrude above body surface allowing fish to see when buried in the substrate;  

About Freshwater "Flounders" of the Families Achiridae, Soleidae, Cynoglossidae, Pleuronectidae. Yes, there are maybe four true freshwater Flatfishes, though strictly speaking they are not true "flounders", as well as many brackish species that venture upriver temporarily. 

Family Samaridae. Three genera, 20 species. Indo-Pacific; marine, tropical and subtropical. Generally deepwater. Dorsal fin originates before eye, lack postcleithra, pelvic fins symmetrical.