The turtles would feast, specially if its a Red Eared Slider. I had one and I do not suggest ever getting a turtle untill you know all about them, takes an awful lot of work, more then fish. I had him in a 10 gal and it is WAY too small. Here is a link to a turtle forum and site, they have a lot of help. They helped me with my turtle and I found him a home. You have to have a least a 75 gal tank, for females, 120 gal 1 turtle alone. And they eat fish. They love Ghost Shrimp and small fish.
Turtle Forum
Please look into it more, because I know the people at the forum are very tough if they think you'll "abuse" a turtle by placing it in a small tank. They need an island, special food, heaters, lights, basking lights, plants sometimes, and a large amount of swimming room, they are also extremly dirty, suggested if you had a 75 gal tank, to get a filter that can handle a 150 gal tank, double your size tank. Most people have 2 filters just for the turtle. They also may carry samanella, though it is curable and you just always wash your hands.
Don't get me wrong, they are fun, sweet, and interesting, but a very big handful. Sad I had to give mine up, but I didn't have the right stuff nor the money it takes to care for them. They also have laws against them, no turtle should be sold under 4 in long because of a mishap back in the 60's. You can read about it at that site. They will help you understand what you need to get started.