it could be depressed, or starving because it
can't eat its food. i made the mistake of listening to the idiot caresheet from the lady who sold it to me, too, and i'm glad i found this place! (you too!) cuz now i can be sure i give my turtles what they need to keep them healthy....
these are what this site has taught me, and it should help
red ear sliders are aquatic turtles, they spend most of their lives in water (even sleep in water) so the more the better. water helps them with various survival traits too: they gain strength in their arms and legs by paddling with their feet to break the water's surface. my little ones are strong enough to shove my finger away! also, as aquatic turtles they lack in salivary glands. that means they need to drag their food into the water to be able to eat it. so those liters of water most likely isn't even enough for them to put their heads in very well, right? like many people here told me when i first got mine: give them more water, they will
love you for it!
hope that helps!