Believe me, I know how you feel.
It's sad... you and her could share such a cool interest in fish together... but she's not open to that right now.
Arguing with your Aunt won't make the situation better... and will just make you feel worse. Lead by example. Either she comes around or she doesn't, but in the meantime, you've done the right thing for your fish... and you.
There is an old saying...."You can lead a horse to water, but you can't make him drink". Think of it this way... "You can lead a man to truth, but you can't make him THINK". It's up to the other person to grow and change... no matter how much you nuture them and help them to see.
There are none so blind, as those who will not see... pity them, for they are the ones who are missing out. Stay open-hearted, though... people change, sometimes when you least expect it...