Yes. A red ear slider can be put in a beta tank as long as the tank is big enough and there is plenty of room for both the turtle and the fish. The red ear slider is a larger turtle, so it will need a tank that is at least 10 gallons or larger. The beta fish can live in smaller tanks, but it’s best to give them at least 5 gallons of space each. So if you have a 10-gallon tank, that would be perfect for housing both a red ear slider and a beta fish.