Howdy,
It is amusing to see the immaturity in conflict solving of many of you who replied. Yeah, slap your mom. That's great advice. Answer rudely. Another good one. That's the perfect way to convince her to have respect for your hobby. Although I hope these advice were not serious, I think a few words of sense wouldn't hurt to go along with them.
I am not expecting anyone to agree publicly with me, especially not after what has been said. But when the computer is off and you're at home, eating a meal your mom has prepared for you. Maybe then you'll think of what I wrote.
Respect your parents and act as what you are: Young adults. Not babies. Think seriously about what your parents say, and you will maybe understand them better. Indeed, an aquarium is an expensive hobby. But there are other expensive hobbies which are much more detrimental, as stated correctly by guppy. Many fish are caught in the wild, others bred merely for the purpose of living in an aquarium. Take your mom to your tank and show her which fish were bred just as pets. Explain to her where they come from and talk to her nicely. You may be surprised, but being nice goes a long way. It completely takes the air out of her arguments. Much more so than blowing up does.
Don't get me wrong, I understand your position very well. I also lived at home and had fish. Venting is okay, and that's what friends and also this forum are for. But be respectful at home.
HarleyK