Ask them if they'll take it back and return your money. And if not, sell it on ebay.
Did you set it up yet?
If it's still perfectly intact, tell them your fish died - and try to cry a bit, they'll take it
You could tell them it was bowing at the front so you were worried about it leaking or summat, beware though they might tell you they can only swap it for you and not give your money back.
If you havent unpacked it and you bought it in the last 14 days UK law (didnt check where u live) states the product is returnable for a full refund.
How old are you? Why do you keep asking questions then ignoring everyones attempt to help you?
Have you been using this tank for a week yes or no?
If you have the shop will not take it back and no-one will want to buy it.
As loko17 said why not keep them both.
It's quite simple, keep them both or sell the un-used new one.