You know, I forgot you said that Soup was wild for a while ... that could very well be the reason for the diarrhea. Since she was wandering in the wild it's possible that the cause is parasitic rather than dietary ...
That doesn't change the fact that you should cut down on the fruits .. LOL
Sugars found in fruits actually help parasites to florish so you may be compounding the issue further without even knowing it.
I've experienced my tortoises being kind of loose when they need to be dewormed. Usually it clears right up after a treatment.
Has the turtle been taken to the vet for a deworming? I don't know if you mentioned that before in one of your previous posts.
The skinniness also leads me to believe that parasites might be the culprit ... how is Soup's appetite compared to your other turtle's?
Don't worry too much ... deworming is a pretty easy thing to do if you haven't done it yet ... Just go to your herp vet and bring along a stool sample. Heck, you can even check for parasites yourself if you have one of those cheap kiddy microscopes ...
The only trick with aquatic turtles is to not reinfect ... that means that if soup gets dewormed then your other turtle has to get dewormed at the same time (so bring them both to the vet) and you have to disinfect the tank at the same time you deworm so they don't reinfect themselves ...
Parasites normally aren't an ailment of aquatic turtles kept in captivity (hence your other turtle's healthiness) but since soup was exposed to all sorts of other things in the wild then it could be a problem for you. And that fruit really could be stirring things up ... well, make an appt. with a herp vet and let me know what happens!