Turtle not using back legs

by Leanne
(Australia)

What is wrong with my turtle, he is about a year old and has been healthy until now... he is not moving much, but when he does he only swims with his front legs. He is not using his back legs at all..they don't seem broken, his shell is also soft,so was wondering if has deficiency or if an extra filter is the problem? I have tried to feed the turtle fish blocks but prefers fish flakes (which he pinches from the fish in tank with him) is this the problem..mind you hes been doing this since I've had him. Please desperate to help my turtle..thanks