Hannah, did you try returning Fighter? I know a lot of stores have a dead-fish return policy. It's horrible to say, but you might be able to get your money back or a replacement if he dies with in a certain amount of time. So not only did you try to save him and gave him a happy, warm home for a little while, but because he passed away due to their treatment of him, you might be able to get your money back.
I read your other post about all the fish your dad got you, and you are very lucky! Good luck with everything!