Before you even think about transporting you must ask first. It would be a shame for you to be stopped at departures and most airlines would not have anywhere to keep a fish.
You would not be allowed to keep it as hand luggage and going into a hold would be dangerous if not packed properly. I'm not sure how they are packed for flights but i would think they would be in plastic bags with added oxygen and then well insulated against the cold as it will get below freezing in their.
Maybe best to take him back to a pet store who will find him a good home.