I’ve caught plenty of Fallfish in the mid teens, and a few that maybe went 17-18”. They can look very similar to Creek Chubs, especially males in spawning season. Size is an easy differentiator though. If over 10” or so, it’s nearly certain you have a Fallfish. If over 12”, it is a Fallfish. Fallfish are a little more streamlined in overall body shape, and have comparatively bigger mouths also.
They’re found in generally similar habitat, though my experience indicates two slight differences:
1. Fallfish are much more tolerant of moderate or even heavy current, and in mid-Summer you’ll actually find them in pretty fast runs and deeper riffles. Creek Chubs prefer much slower water, all the time. They’re actually pretty common in small wild Trout streams in PA too. You’ll often catch Creek Chubs from the slack frog water at the bottom of a pool in a small Trout stream, and find the Trout at the head where the current is swifter. You rarely find Fallfish in small wild Trout streams.
2. The larger the size of creek or river you are fishing, the more likely it is to find more Fallfish, and less Creek Chubs. Most of the waterways being discussed in this thread most certainly have far more Fallfish biomass than Creek Chub biomass.
I’ve learned to enjoy Fallfish. They fight well, eat dries, and on WW streams they often still bite on days when the Smallmouth seem to have lockjaw. Creek Chubs I still see mostly as annoyance, other than serving as nighttime snack opportunities for Pool Boss Brown Trout.
Both species, and the entire minnow family in general, have been proven to be effective Sasquatch repellent.