"Freshwater" flounders (Achirus sp.) are brackish to marine (though they exhibit a broad range of salinity tolerance, especially when young, when they often ascend rivers) and coldwater; when adult, some specimens may reach lengths in excess of 16 inches. They are difficult to wean off live foods (they prefer brine shrimp, daphnia, and small "worms" at the size most aquarists obtain them - 2 to 3 inches in length), but some have had success with feeding them frozen and freeze-dried meaty items. They will never thrive in a tropical freshwater community, and should be provided a sand-based substrate, as they become stressed out and "frayed" over gravel bottoms - this substrate choice precludes both burrowing and proper callibration of their chromatophores as a camouflage measure (especially so over mottled "natural" tones).
I believe there are Peruvian/Amazonian and Australasian true freshwater soles, but they are hard to come by.