Pop it in the freezer for the time being while you decide on how to preserve it; that will help to stop it from decaying.
One option is to completely submerge it in either formalin (formaldehyde) or isopropyl alcohol (rubbing alcohol) and sealing the container that it's in. With this method, you may need to make slits and/or holes in the body of the fish in order to ensure that the preservative is able to fully penetrate the body. Also, you'll have to replace the preservative after a few weeks because it will start to turn yellow; you should be fine after the initial replacement of the preservative, and it should stay fairly clear after that point.