I'm going to get my chops busted bigtime for this one, but there's a big disclaimer. In a 20g, you *could* do a tang, but for such a short amount of time, you probably wouldn't get much enjoyment out of the fish. I've noticed a trend in the LFS lately of every now and then getting in some seriously small tangs. I've seen yellows the size of 50-cent pieces and hepatus blues less than an inch long. If...IF...you got a healthy specimen (and I've *rarely* seen a tang that small look good), a 20 g could handle it, but probably for only a few months until the tang got even slightly larger. Then he'd either have to go back to the LFS or into a larger tank.