Firstly, we're talking Southern Flounder here, Paralychthys lethostigma , the flat critter that stalks shrimp and baitfish movements from tacticly chosen positions on the bottom of warm bays (and nearshore Gulf bottom in mid-winter) from the Atlantic Seaboard in high summer, all the way down the Gulf of Mexico to, about, well, Northern Mexican waters.
They are geniuses of the ambush, staking out positions along edges and entrances where bait and shrimp would naturally travel with the least resistence against a moving tide, and along shorelines where these prey items seek refuge from them and other predators.
Forgive the lack of focus. My cameras are 'tards when it comes to velocity of focus on a thrashing object. I'm working on that. Slowly.
They eat flies like candy. It's simply a matter of putting the fly over their roving eyes. Hit the bay locations like a good chess player, thinking "ambush", and you'll score in transitional months and also in the heart of summer, when the fish are concentrated in the bays.
Flounder are, in a word, "unexpected". They strike when I'm on the redfish flats and blind-casting while I'm bored because I'm not seeing tails. But then there's the explosion of the airborne flounder flopping back onto the surface of the water after exhuberantly taking a morsel from underneath at great speed and continuing on up into the air, a foot or more sometimes, to fall back down and smash intentionally flat onto the surface, just to make a public display of his success that moment, it seems. Gotta' like the attitude!