Freshwater flounder
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Freshwater flounder can be found in freshwater springs (completely non-brackish) in Central Florida. I saw hundreds of them over the course of a decade of scuba diving, so I am quite certain that they exist, but have only been able to find articles on river dwelling species which migrate to brackish waters later in life. This species is similar in color patterns to Achirus lineatus, but more elongated. Peacock Springs, a series of interconnected springwater caves, is a great place to find them.