Well, actually they do. They live primarily in termite mounds and pre-dug burrows. So confinement is in their nature.
20 Gallon is still too big in my opinion. Yes, some bps will do fine in a large tank, but if it's a problem feeder then the first step is to get it into a small 10 or 15 gallon tank or a 15q tub.
Petstore employees tend to give very questionable information.