Everyone makes mistakes. Many people buy fish they aren't prepared for. Miah, I recall a few of your mistakes from the bettafish forums, and there is no reason to be so darn harsh to LLL. None at all.
LLL - in your case I would try to get the biggest container that will hold water that you can, put the fish in the garage or even outside if you won't rehome him.
Why does his initial cost negate the fact that he is alive, a living, breathing, feeling being? Yes, it is a .15 feeder fish, yes it might have been food for another fish. But that would have been a quick and painless death (most likely), vs having to sit in a toxic environment and slowly being burned to death :( (Not being mean, these are the facts. Ammonia will burn and poison the fish. He may survive in your 2.5 gallon tank for now, being small, but he will grow and grow and grow. And produce a LOT of poop, thus causing the ammonia problem. Goldfish are notorious for being messy fish and producing a ton of waste. And when he grows, he may stop, but his internal organs won't. His spine may not, causing him to be deformed and live a poor quality of life.
But in the end, it is your fish, and we can't make you do anything, only provide facts and education.