Turtle not acting himself..

My friend has a spotted turtle; I am asking this on her behalf. She says that lately, this little guy just doesn't look the same; lackluster, kind of limp and just not "himself." Could it be parasites? I thought of suggesting deworming but now that I think of it, seems I heard you can't use ivermectin for turts? Is this true..? He is caged in a big 45g tank, half water with whisper filters running and some pretty old unfertilized potsoil as a substrate with plants; naturalistic setup.
She does have a permit to own this protected species.