About 15 years ago I bought some goldfish from a garden centre near Doncaster and noticed in the tank some enormous tadpoles. The proprietor explained that the fish were bred in Florida and occasionally these tadpoles arrived in the consignment with them. I managed to get him to include 3 tadpoles with my goldfish purchase which I subsequently reared in a small aquarium. When all the legs had appeared and the tails absorbed I realized that they were American Bullfrogs.
Even as juveniles they were about the size of a year old common frog and were capable of eating enormous amounts of food (they were particularly partial to steak mince as well as copious quantities of woodlice, craneflies and other assorted insects).
The only drawback with them was the racket they made all night. Chirping so loud it could keep you awake.
As they grew to larger proportions (too big for their aquarium) and realising that to release them into the wild would be an act of potentially catastrophic environmental stupidity I sold all three of them on to a French restaurant.

.....seriously, I had them taken away by the RSPCA who found them a home in a zoo somewhere!