Turtles and fish should not be kept together as a general rule, even if the turtle does not eventually kill and eat the fish, turtles are very messy eaters in general and produce a lot of waste which sinks to the bottom of the aquarium where the pleco lives- this builds up and will cause either ammonia or nitrite poisoning or both, killing the fish (sometimes this happens very slowly, other times very fast- either way its not a nice way to go).
I can't spend long here, but long story short, there are a variety of reasons why turtles and fish shouldn't be kept together in general. What sort of turtle do you have and what sort of environment are you keeping it in?