As a parent myself I can see two differnet points here.
First be totally up front with what you want to do. Back doors will only work for a very limited time frame and you will lose trust for anything you might want to do in the future.
I would suggest doing some research, find out how much the devices attached to your tank draw in electricity, with the heater you might need to take say 3 readings morning afternoon and evening (for an hour) to show how long it is on for and then calculate the average hours per day it will be on for.
Take these values and then find out the cost of electricity in your house, present the facts, my tank costs £2.24 per week to run (for example) and I am willing to clean the car fortnightly to cover it. Cleaning the car won't seem so bad when you go back in and look at your fry!
They will hopefully appreciate the effort and realise you are keen on what you are doing, as mentioned earlier explain the learning in your hobby, are you just keeping the fry or trying to breed the best ones, therefore getting a basic understanding of genetics, without knowing your age maybe you could ask a science teacher at school if they have anything that could help your understanding of breeding.
If you have access to powerpoint or the open Office equivalent put a presentation together, a good learning skill and it has won discussions for my kids when they propose it almost like a business proposal.
And I understand your problem, I could think of a couple of uses for some more tanks but I am banned, (the wife) from getting any more, though I hope to work on that one also!
Good Luck
StatMan