Your parents sound very weird if you dont mind me saying.
You should try and convince them by telling them you are going to get a job to pay for it. The best thing to do is rather than trying to bribe them, show them how determined you are in getting the tank, and that the interest in fish is not just a passing phase.
The best place to start off making the odd bit of cash are things like babysitting, cutting people's lawns, washing cars, etc. Try down your local shop, see if they need any help with anything, if you have a LFS close by, you can ask them if they need their tanks cleaning maybe?
Just remember, if you can show your parents you are this determined by doing horrid little chores, then they may end up helping you out.
Ben