Ha ha. You're turtle would ADORE those fish.......for lunch.
A red eared slider is not a pet to just throw in a tank. They require 10 gallons per 1 inch of shell length. Since it is illegal to sell or purchase a turtle less than 4 inches (number may vary by state) this means you would need to be starting out with 40 gallons for a baby-and that baby is going to grow fast. Turtles also require very heavy filtration and much more frequent water changes than fish. They also need a place to come entirely out of the water, and a basking light with uvb. Please do your research if you are serious about getting an res.