"Tank busters" are big, powerful fish unsuitable for the average domestic aquarium. If you've seen a species on TV being held by a celebrity angler, it's fair to say it meets the criteria and some of these are all too commonly available. Some of these mighty beasts, such as the big catfishes, are predators with enough strength to crack a tank with the muscles evolved for fighting river surges.
Many of the more commonly encountered tank busters are food fish, mass-produced on fish farms for growing-on in ponds. This means they're very cheap and can tempt the unwary when encountered as cute babies in stores that should know better. Pangasius catfish are a great example of this and can be found in unethical fish shops as well as supermarket fish counters. Like all tank busters, this fish makes a beautiful and long-lived pet in appropriate conditions for those able to provide them. Realistically, these facilities are rare outside zoos or public aquaria and they struggle to re-home fish that may live thirty years and grow a metre or more in length.
Sadly, as our systems include larger stock tanks than many other outlets, we often find these homeless fish on our doorstep, or rescued from customers who either didn't do their research or received poor advice from other retailers. Take a look at the Big Fish Campaign for details of the most common species in this category.