One thing that might honestly work, hon, is telling them that you want to persue a career in biology, and having water-bound creatures to study at home will greatly increase your knowledge.
You never know, you might be able to write all kinds of papers and do projects with/on your tank inhabitants.
I would think that if they are really into you doing well in school, try to use that to your bargaining advantage.
All fish have Latin / Scientific names, and learning these will help further your foreign language skills. You learn the root of a word, the root of a type of fish, and you can figure out a lot of its base needs.
I would really just try to wrap it around school.
When my parents didn't want me to get a snake I acted like I had given in. One day while they were out I moved a terrarium into my room and had it sitting out, empty. I never said anything about it and they ignored it. One day they come home and BOOM!! -- there's a snake in there with all the bells and whistles of reptile care. They were shocked. They knew telling me to get rid of it was not an option (being a girl comes with that innate ability to cry when occasions call for it; it's sweet), so they had to deal and I had my snake.
I don't know if something like that would be possible.... but I have always been a strong believer in the fact that they can't reprimand you for having something if they don't know you have it =D
Good luck, hon!!! Let us know how it goes, yeah??