Stunting isn't exactly a good thing...
And they don't get stunted easier than most fish, it's just the percentage that are kept in adequate water quality are really low.
Most die soon after they're bought, much like yours, the ones that grow larger end up killing themselves by injuring themselves on things they bash into (this happens when you keep such a fish in too small a tank), getting sick and dying from the infection of those injuries, or do not grow to very large because they were kept in poor water quality their whole lives.
It's really bad for a fish to stunt it. The body stops growing but the interal organs don't necessarily stop growing.
The other thing about buying fish is that you should never buy fish depending on their potential MINIMUM size, you should buy fish depending on their max size or at least their max size in aquaria. You can't buy a fish HOPING that it only hits the min. size.
And you say the largest you've seen is 18"? An 18" ir. shark still isn't exactly the best candidate for such a small tank since they're so active. Of course, they'd fit, but it wouldn't be the most appropriate housing for them.
