Koi or Comet?

Question
Chris....
you know how I was saying I wasn't sure if the bigger fish that I got was a common Goldfish or a Comet...?  Well...umm...I did a little bit of research online, and well...the little japanese style whiskers it has are called "barbells."  Goldfish and Comets don't have barbells.
...Chris...I think I bought a Koi!!  It's true that I paid only $4 for it because the sales lady probably thought it was just a goldfish when it was supposed to be $15 or more for a Koi.
It's 4 inches now, but it's supposed to grow to 16-24 inches, I read.  What do I do?
I have to return it to the pet store if this is the case.  I'm new at all this.  The aquarium is hard enough as it is...I can't fathom going out to my backyard and digging a 2-4 foot deep POND and fill it with 300 gallons of water and set up a filtration system for it toooo!!!  Do you know anybody who'd buy a Koi for a goldfish price?  Thanks in advance for your help.

Answer
Hi Basil;

Oops. You are quite right. You have a baby koi. That happens sometimes. They are even mixed with the goldfish by accident occasionally. Koi are jumpers too so they will often leap into the wrong tank if given the chance. I would take him back to the store and let them know. Just make a daily 50% water change until you can get the koi and goldfish out of there. Ammonia is burning them so they really need some relief.

Good Luck....

At Your Service;
Chris Robbins