If I may... A polite word with the staff will go a long way with facilitating a return or exchange. Everyone makes mistakes. If you are polite with the staff and they still won't accept the fish for a return, then I would get a little less polite.
It's gone back, probably be in another 10g by tomorrow though, I started off all nice and friendly, I said I wanted to return the fish because it wasn't suitable for my mums tank, and that's where it all went down hill...
"If it wasn't suitable why did she purchase it?"
I saw red... But stayed calm explained that it was a member of staff that gave her the advice but after looking into it it's not suitable for a tank of her size.
"Then you really should get a larger tank, we can't refund the full price of the fish because it's been exposed to your tank and may die when we QT it, so I can offer you 50% store credit"
That's when I lost it...
I told her with a few choice words "that if she bothered to train her staff we wouldn't be here, I told her that my tanks are cleaner than her shop, I told her my water is most probably of higher quality, and finally I TOLD HER THAT I AM getting a full refund, and store credit is useless because I wouldn't be shopping here again.
Needless to say I got my money back.