With captive-raised aquatics, if they were kept in the water, the nails usually aren't any longer than wild turtles. If anything, I've had it the other way around, where the nails of male painted turtles failed to grow due to improper husbandry.
I say put in another turtle, and see if it "flutters" at the other turtle. I've never seen a female slider flutter. Do you know how old these are, or what incubation temperatures the breeder used (if you're getting in large groups, I assume they're coming from a breeder?)?
Katrina
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1.2 Eastern Muds - Fred, Ethel, Edith
0.1 Iguana - Tiffel
0.1 Bearded Dragon - Foster
Foster turtles: More than I'd like the husband to know about.