Sorry to be a boring old bastard, but somtimes you gotta respect your parents wish's/rules depending on the situtation.
IF you are paying for the tank and all the gear to go with it and the fish etc and it is going in your room, fair enough- you have every right to get it in my opinion.
But i can understand your parents not been so keen on you getting the tank if they are paying for most/all of it or its going in a family room and they don't want fish tanks in every room of the house or that particular room or they are somhow going to be involved in helping maintain the tank.
Despite growing up on a farm, i wasn't allowed any indoor pets and my mum got angry enough when one day i turned up with a small goldfish tank despite me paying for and maintaining the thing and being in my bedroom. I'd feel very lucky and grateful at the time if my mum let me have 6 rats or 4 tanks or whatever.
On the other hand i knew the sooner i got my grades and moved out and got a job the sooner i could buy as many and whatever types of pets i wanted, so i gave me alot of motivation for doing those things and getting them done.
Now i am renting my own apartment and hopefully getting a house soon, i have good jobs and are very independant.
I think good ways to persuade parents to let you have a new tank is the promise of working realy hard and getting good grades at school/colege/uni or working/doing some household chore in return for them- that way everyone benefets and is happy and you get your tank. Simply buying the tank and fish without them knowing though is kinda sneeky though and not exactly respectful/responsable behavior...
Okay so im old fashioned i know but if you work hard in some way in return for getting the tank i think that is much better
.