Gave up on them today - picked out all of the shells and put them in a shallow tupperware with some cold tap water...
Lo and behold, the two or three really big ones (like 1" long) started moving. They moved into my 10G Molly Fry Containment Unit for the moment, where they seem happy.
So now I'm wondering, why don't these snails like my 110 tank? Nitrates are high-ish because of EI dosing, could that be it?