Its all about how they swim, although flat fish don't have a rattle as far as I know while most kwikis do. Personally I don't like rattles anyway unless the water is super dirty. Even if they look good they may not be fish killers, spend a season rotating em out, it helps when you know biting fish are around. I've spent years perfecting my supply of kwikfish, after buying a couple dozen I have 5 or 6 that are my number ones. Put a file mark on them or name them on the belly to keep track of their success.