Well....
1. How big are the fish? If they are bigger then the turtle, lets say a pleco..., then thats not a good idea, since the could poosibly harm the turtle.
If the fish are smaller than the turtle, then the turtle will most likely eat them, due to the fact that all turtles are omnivores and will eat fish, invertabrates, crustaceans, etc.
Do you want dead fish.....
If not, then take the turtle out, and give him his own 50 gallon aquarium, filled 5/6 of the way with clean, filtered, heated water.