Yup... I'd have to agree with the others. Your arrows are likely too stiff. Where are they impacting the target? If they are too stiff, they'll likely be hitting left of point of aim.
The other issue with fishtailing is a crappy release... plucking the string, etc. Only way to resolve... practice, practice, practice. Don't over exaggerate your release... just let your hand relax, let the string slip your fingers and follow through a bit. My hand usually ends up around the bottom of my ear after release.