Well if it's your little brother's fish, just talk to him first. "Do you want to see Puffy HAPPY and in a huge tank? We can do that, okay? He'll be warm, he'll have place to swim, he'll be the coolest fish on the block. How bout it? Want to get a bigger tank for him? You can come with me." Sorry, I don't know how old your brother is, haha. This is through assumption that he is quite young. If not, change it if you need to...just a suggestion. If your little brother said "lets get a bigger tank", will your mom be okay?
I would say wear her down (I'm living under my parents roof as well, I've been through this a number of times, haha). But honestly, if it is your money, then it's your money... return the 1/2 if you can. Is she a sucker for hearing "mom, it's a fish...it's just inhumane sticking it into this box." (or etc)? Mine wasn't, but I showed my dad the 5 gallon for $30 bucks, and he was just like "woah." And went ahead and bought it. My mom stayed silent. Also, if you heard from an employee the bettas are fine in the little tank, just say "they're just there for a job, not the fish." This is often true.
I would really recommend doing a lot more water changes. It's true he may get stressed, however... like 1f2f said, he will die faster from ammonia levels than stress.
Get a bigger tank AND heater. I believe these are both equally important in this case. However, if you really must know (lol), I say a heater is not even a centimeter behind a larger tank. Filter last, although it may come with a tank anyway.
My little brother is 5, so I'd probably have to talk alot to him...
But, I have a feeling she'd not going to budge, even if I get my little brother on my side. She's got 4 children, and I'm her 'animal loving child'. I've been trying to wear her down for the past 15 years of my life, and she's heard every excuse in the book and my head. Plus, she's seen how I get when I have hamsters, and when she lets me improve their cages. Usually it ends up with elaborate cages that take hours to clean (no joke). I have a feeling she doesn't trust me to improve the family pets' cages anymore... ^^;
Unfortunately, she's not a sucker for the compassion act, or the the fact it's my money. Like I said, she bought the little tank for way more than it should cost, so that's got her stuck. I'm not sure Petsmart will take it back, but I'll have to see. I've tried asking her, and explaining that it's possible to get larger tanks really cheap, but no, she's not really listening. Right now, it's to the point where if I suggest one more time in the next week or so, she'll probably refuse to even take me to the petstore to get more water conditioner. (I'm hoping if I can get her to the petstore, and into the fish section, I can persuade her to let me buy a larger tank)
Right now, I'm cleaning his tank once a day, and I'm not netting him, but rather trying to have him swim into the cup, so that's decreased the amount of stress.
If you want to try to persuade her by taking her to the fish section, definitely go to Walmart. 5 gallon + filter for 30 dollars? Again, that totally bought my dad. Lol. I see 2.5 gallons that are $25...and other 5 gallons are $45 usually as well.
Darn with lack of compassion. That was my mom's problem as well, but if I managed to get it on my own (friend drove me, I bought with my own money, etc), then she couldn't care less. Or, maybe, she did care, but it passed with time.
Actually, we just stopped by Walmart during a car ride... but she still said no. But, I just now realized how stupid I am XP My dad owns a business that sells a lot of plants, growing stuff etc, and thinking about it, I'm pretty sure we have a few little 5 gallon tanks around the store, and knowing my dad, he might let me have one... I think.
@Iamtetsuo: thank you for the advise, I'll try that if asking my dad doesn't work.
@xxabc: I think it should work, he said he'll consider getting me one if I help him with the plants he's growing at home for the store. :D We don't have aquatic plants, but i'm sure I can find some.
(I did a bit of research, and they sound so cute, if just a bit hard to care for :) )