How can I convince my parents to let me get a bigger tank! 20 + gallon hopefully?

I have a small tropical freshwater setup in my room, only 6-7 gallons, which has 6 guppies, 2 dwarf gouramis, 1 Otocinclus and 1 Sterbai Cory, so its pretty over-stocked. I know also, thats a pretty weird looking array of fish, but I love them. Ideally, I am hoping to get either for my birthday or next christmas,... show more I have a small tropical freshwater setup in my room, only 6-7 gallons, which has 6 guppies, 2 dwarf gouramis, 1 Otocinclus and 1 Sterbai Cory, so its pretty over-stocked.
I know also, thats a pretty weird looking array of fish, but I love them.
Ideally, I am hoping to get either for my birthday or next christmas, or buy myself, a 20+ gallon tank. If I got that, I would add one or two more Otos, and 3 more Sterbai's. Or, I would get something larger, like 50 gallons (^.^ !!!) and add a few more other species. I asked my mum about it, but since I only set up my current tank maybe 10-12 weeks ago, she says things like Why did you waste your money on the stuff you have now then, etc and stuff like that. Stupidly I relied on my tropical fish store, which told me my tank would be fine (argh). I already had the tank, but needed all the tropical accesories; all up it cost me $100+ for the heater, filter, food, accesories, chemicals etc.
So I need to know a few things:

1. How can I convince her? Maybe also if I could have it downstairs, because of the weight, and how pretty it really is to have fish.

2. If i got 50+ gallons, what other fis should I add? I really like Angelfish, btw.

3. Thanks!
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