Sometimes, you just have to accept what your parents say. Don't expect instant gratification for being good. Try your best to be understanding, sometimes parents will get overwhelmed by pets, even though they aren't the ones taking care of them XD
If possible, in a few months, slowly start collecting the things you need for a tank. Maybe just one thing here and there, then one day, just look around for a good spot, and set the empty tank there. Chances are, if your parent's notice, unless they say something, they're already halfway to accepting the fact you're getting another fish. :)
If they do say something, just mention the fact that you have everything for a complete tank. If they ask you to move it, find another spot, and set it up there. Eventually, they're bound to accept the fact you've gotten another tank ready for another fish.
I'm sorry you couldn't get your dream fish when you saw him, but another one will pop up in your path again, when your parent's are willing to say yes to the fish. :)