Question about a Bala Shark I bought yesterday?

I told the lady at the pet store that I have a 20 gallon tank, I told her I needed a fish that would fend for himself and not get picked on. She said that I should get a Bala Shark so I did and found out that this thing can grow 1 foot. She never told me that, even when I said I have a 20 gallon tank! I want to take it back but I don't know what to take it back in. What should I use? Also, do I need to take it back? I know that if you keep big fish in small tanks their growth will be stunted. How fast do they grow?