I remember many years ago when I worked in a pet shop a woman brought in a channel catfish about 18" long, it had been in her goldfish pond as a "scavenger" the owner of the shop gave her store credit for the fish and told me to put it in a 100 gallon tank he used to sell goldfish out of. The tank was full of fancy goldfish, I tried to tell him it was a mistake but his response was that the fish had been living with goldfish for years and it only ate pellets. So I dumped him in the goldfish tank, he ate a dozen of them before he got to the bottom, it was so funny...