Ah haha- not all "freshwater flounder" are brackish. Most flounders bought in chain LFSes are brackish, but mine is true freshwater. Check with these fishbase pages:
http://www.fishbase.org/identification/specieslist.cfm?famcode=440&areacode=&spines=&fins=&c_code=
http://www.fishbase.org/identification/specieslist.cfm?famcode=516&areacode=&spines=&fins=&c_code=
Click on the species that looks most like your flounder. Once you have a few species that you think are possible matches, click "Pictures" in the "More information" section to see more photos of that particular species.
When trying to ID a species, look at which side of the fish the eyes are on, where the fins stop and start, how many rays are in each fin, any strong patterns on the fish's skin... Don't pay HUGE amounts of attention to the actual color of the fish - most flounders change to match their environments, so that's not going to be particularly helpful.
If you can, take some close-up photos of your flounder, post them, and we can try to match him up with a species.
Just to note, these are common species in the aquarium trade:
http://www.fishbase.org/summary/SpeciesSummary.php?id=4256
http://www.fishbase.org/summary/SpeciesSummary.php?id=4260
and this is the species of my flounder:
http://www.fishbase.org/summary/SpeciesSummary.php?id=57094
See if any of those help you.