Cezza - By "mane" do you mean his gills on the side of its head or the fin that runs down its back?
Fish should never be left in the tank with axolotls, as fish nipping an axolotl is common. I trust you have removed all the fish from the tank?
You will want to watch for signs of fungus on the damaged area. You may want to give the axolotl a few salt baths.
A salt bath is 2 teaspoons of salt dissolved in 2 liters of water. Soak the axolotl for no longer than 10 minutes. You can give salt baths twice a day.
If the axolotl is badly damaged I would recommend putting it in a food safe container with lid (like an ice cream bucket) and putting it in the fridge.
Keep extra water in the fridge to change its water with so you have water that is the same temperature. Keep its water clean and offer it food daily.
You may want to tape the light switch down in the fridge or cover the container with a towel so the light does not frighten the axolotl.
I do not recommend using feeder fish from a pet store. Feeder fish are not usually taken care of well and can carry parasites and other problems that can be spread to your axolotl. If you feel you need to feed live fish I would recommend you grow your own at home to help insure they are parasite free.
There are many foods that axolotls will usually accept; earthworms, crickets, axolotl pellets, salmon pellets, grasshoppers, slivers of fresh fish fillets etc. All of them much safer than live fish.
If you have not already found it you may want to visit caudatas sister site
http://www.axolotl.org/index.htm it is a must read for axolotl owners.
