Its hard to believe based on how little they are at the store. But MAN, I remember the first time I saw a group of full grown bala sharks. I was BLOWN AWAY. They are HUGE. And since they need groups to be comfortable, it makes it very hard to accommodate them in a home aquarium. The tank they were in was at least 180-200 gallons and the group of just 4 of them looked cramped!!!!!
That is so sad to me that they would suggest a 20 gallon for a fish that gets 12 inches. Forget the need for a group! No 12 inch fish should live their lives in a 20g. They will get severely stunted and die very young if kept in a tank that small.
I am glad you are willing to take the advice even though it is not what you wanted to hear.