In general. the FW flounder that is most often found in fish shops in the US is the hogchoker, Trinectes maculatus. THis is actually a type of sole rather than a flounder. Though these are found in pure frshwater, sometimes hundreds of miles inland, they are still a brackish water fish and would do much better with the addition of some marine salt. WHen small, they will eat live black worms, and then possibly frozen blood worms. THey need something meaty that will sink to the bottom. As they get larger, they become more piscivorous in nature and would prefer small fish or earthworms. These guys get about 6-8 inches long and really should be kept in a species only tank if possible since they generally aren't very aggressive feeders. They should also be kept in a tank with a sand substrate. They prefer to spend the majority of their time partially buried in the sand with just their eyes above. These are very interesting fish to keep, but they are certainly not for beginners, nor are they particularly easy.