You can occasionally give her other types of fish but try to avoid sea fish. You should totally avoid giving your turtle fish that has been previously frozen. This fish will inhibit the absorption of certain nutrients and might cause her long term problems. You can buy them at most pet shops since they are the same used by birds. Your turtle refuses to eat If your turtle refuses to eat an item that she previously enjoyed eating, don't worry.
Sliders are finicky eaters and sometimes just want something different.
If she refuses to eat one item, offer her different items. As far as she is eating 'something' there is no health problem to worry about. Suspect sickness only when she refuses to eat ALL food items that you have offered.