Sorry to hear about your fish,
What kinda of injury does your fish have to his/her head?
Also if the fish tends to swim on its side then this could be swim bladder disease, which is best dealt with earlier than later.
The swim bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, the most common cause of swim bladder compression is a distended stomach from rapidly eating, overeating or gulping air.
My comet had this problem last year, all you have to do is give it a few green peas that have there outer shell removed, and depending on the fishes size, cut it in half or quarters.
Also my other common goldfish had a cut on it a few years back and I was told to do lots of water changes and the fish will heal by itself, which worked perfectly for me.