there is a fish titled "freshwater flounder" but it requires brackish.
It's inaccurate to describe all species of freshwater flounder as requiring brackish. There's more than one species, and not all are brackish.
Trinectes maculatus for instance, the common hogchoker (So called because they literally choke hogs when mixed in amongst other foods) is a brackish fish. However, i keep a
Brachirus harmandi, one of the few true freshwater species.
There are plenty of varied types also (not all available in the trade), check fishbase.org for more
Brachirus annularis
Brachirus muelleri
Brachirus orientalis
As you can see, plenty of different types!
Finally, a photo of mine attached to the glass.
I did recently see on another forum photographs of a "giant freshwater sole" which was huge, larger than a dinnerplate, and allegedly collected from peru. There are plenty of types, they just need careful ID.
If you have a photo of yours, i'm sure we could ID it, you can have a go with this site as well (the best site on the net that I've found incidentally) for ID -
http/www.geocities.co.jp/AnimalPark-Shir...kareiindex.html