sounds like they are worried another fish will overwhelm you. you will just need to take extra good care of your current ones and show them that you are not overwhelmed with fish care + school+ whatever else you spend your time doing.
and fwiw, i am a huge fish and pet lover, but if my child sneaked a pet in my house w/o my permission? there would be consequences. so I DO NOT recommend that route at all.
sometimes parents seem to have no reason for the things they won't allow you, but trust me there is always a reason. i believe they should share with you their concerns, but in the end it is their house. one day when you are older and have your own house and kids you will understand. trust me, i get how frustrated you are, i well remember it when it seemed there was no reason and my mom just said NO! but if you want to maintain their trust, please don't sneak anything in.