Thesharpestlives: What I am about to suggest is not the smartest thing in the world. It will probably get you into a lot of trouble,quite possibly grounded but if you're willing to take the risk here's what I'd do if I were you(I was a rather rebellious teenager after all)
Just walk up to your parents,tell them that enough is enough and that you're taking matters into your own hands.Tell them that you are taking the fish,plain and simple.I know this is much easier said than done but everyone has to grow up sooner or later and I'm assuming that you're around 15 I'd say now is as good as any other time.
Tell them that you cannot watch this fish die,tell them that you've cried over the situation.You should use the argument that I gave my mum when my school was threatening to suspend me over my protesting against the iraq war(it's a long story) It goes like this.You raised me to be the best person that I can be.You raised me to use my conscience and intelligence,to be kind and thoughtful,and now that I have grown up and want to fight for the things that I believe to be right you want to punish me? Do you want me to be the kind of person who sees cruelty and injustice but does nothing? Do you want me to walk through life oblivious,caring for nothing or no one but myself?Well I'm sorry but I just can't do that.I can't watch something die and know that I did nothing,maybe you can but I can't.I'm taking the fish,you can stop me if you want but when it dies that'll be on your conscience not mine.When I gave my mum this speech(which I'll never forget,obviously not exactly the same but you get the idea)She went from furious monster to blubbering proud mother,she burst into tears and said she'd never been prouder.I don't know what your parents are like but I think all parents like to believe that they've raised someone who's kind and willing to fight for whats right.Truthfully it may not work but give it a shot.Sooner or later all children must make decisions for themselves and defy their parents,it's part of growing up.Phyc yourself up,take a deep breath,walk out there and be honest with them.At least this way you know you've done everything you could and your conscience is clear.Good luck!Pus you haven't done anything subversive like tricking or bribing your brother,you've been honest and up front about the whole thing.
P.s If they refuse then sadly I would suggest abandoning helping the fish.You have a point to prove.Better still save up,buy your own betta and show them how it's done,when they can see the difference between the two tanks they maybe more inclined to give it to you.I know this is crazy advice to give to someone so young but I believe kids need to be told that it's o.k to defy their parents when it's in the name of doing something that's right.
Be proud of yourself,you seem like a damn good kid.