It's safe to use tap water in your aquarium for your yellow belly turtle (slider) if you place the tap water in a container and leave overnight at room temperature.
You can use tap water as make sure that it isn't hot. You can buy a water conditioner in the so it gets rid of the harsh elements. After this is will be safe for your turtle to drink.
Tap water shouldn't hurt them, really. It is okay, of course, to use water that has been purified and ridden of chemicals placed in our tap water for drinking purposes.
You can use tap water that you then treat with the right type of chemicals. If you go to your local pet store they will be able to help you get the right water treatment for your turtle and the tank size you have.
