Yep, everything on shrimp and fish that fits its mouth will end up in its stomach.
Also, flounders are ambush or sit-and-wait predators, so fish will not be able to learn to stay away from it.
Plus they get large and require tanks with a large footprint and an appropriately sized sand zone with fine sand.
I kept some babies of North Atlantic flounders that I caught on the North Sea as bycatch. They are fun for a while, but usually, all you see of them are the eyes that peek out of the sand.