How much water is needed in a tank for a turtle?

It depends on the turtle and the tank. Sliders and red-bellies are deepwater turtles; mud and musk turtles seem to prefer shallower water. Put in as much as it takes to keep your equipment working properly but no so much that the turtle can climb out. It isn't a major problem, as long as the turtle can be completely submerged when it wants and completely dry when it wants.