Auto leveling help

Hi I'm making a fish game and I'm trying to have the fish be able to self right its self over the z axis. Currently my problem is it will lock the z rotation at and it can't be altered without returning to 0 instantly. My goal is to have the fish be able to self right over the z axis over time after being altered. It must also keep its rotation over the x and y axis.

Blockquote

     private void Rotate()
     {
         //calculate rotation
         float rotateAroundY = yawLookInput * rotationSpeed * Time.deltaTime;
         float rotateAroundX = -pitchLookInput * rotationSpeed * Time.deltaTime;
         float rotateAroundZ = Mathf.LerpAngle(transform.rotation.z, 0, .5f);
         //rotate
         Vector3 rot = transform.eulerAngles;
         rot = new Vector3(rotateAroundX + transform.eulerAngles.x, rotateAroundY + transform.eulerAngles.y, rotateAroundZ);
         transform.eulerAngles = rot;
         //transform.Rotate(rotateAroundX, rotateAroundY, rotateAroundZ);
     }