Align to Spline with an animated Fish character

Hi there,

I'm having difficulties with an animted fish I have done. I bought a rigged fish from turbosquid and then set bone movements in place. I have a good fish movement but now comes the glitch. I want to attached the fish to a spline so that it follows the spline and moves on it correctly.
My first problem is that even if I have the object centered correctly, as soon as I attach it to the spline and use tangetial, the fish is starting to rotate in all directions. I then tried auto-keyframing every rotation on the spline but I got tricked into having problems and having to turn my fish over almost everyframe where the spline turned.
My second problem is that the fish stops moving, like there is a hierarchy of the spline tag over the fish movement.

My question is: How can I make the fish keep it's initial movement (fish tail etc.) and make it follow the spline? Then also how to avoid having to keyframe all the rotations?

Thanks for the help.