“When you feed a fish,

never feed him a lot.

So much and no more!

Never more than a spot,

or something may happen!

You never know what.”

Do you recognize those words? They come from the book A Fish Out of Water by Helen Palmer. The man who says them is selling a goldfish. The salesman has an appropriate name—Mr. Carp!

In the fantastical tale, the boy who buys the fish does feed it too much. The fish grows so huge that it barely fits in a swimming pool. That kind of thing could never happen in real life.

Or could it?

When most people think of carp, they imagine the greenish brownish fish living in lakes, rivers, or muddy ponds. But you may also find a carp in your fish bowl.

Goldfish are a kind of carp. They come from East Asia. Yours may measure only a few inches long. But what would happen if you set it free?

It would eat. And eat. And eat. In the end, it would devour an amount of food way larger than Mr. Carp’s recommended “spot.”

“They can eat anything and everything,” biologist Christine Boston tells The New York Times. People do set their goldfish free, often. So Ms. Boston works to stop invasive goldfish in Canada’s Lake Ontario.

A fish that eats a lot grows. Wild goldfish can grow up to 16 inches long. Their huge size prevents predators from swallowing them. Overfed goldfish can also have lots of babies.

Ms. Boston and her teammates also implanted trackers in the big fish. These spy fish led them to larger groups of fish.

A word to the wise: Don’t throw your goldfish into a pond or river to eat and eat. Something will happen—and now you know what.