Question
I'm thinking about setting up a small aquarium. Id like 1-3 koi / goldfish. I've been told if the aquarium is small it will "stunt" their growth. Is this harmful to the Koi / goldfish.

Answer
What normally happens when you put fish that grow large in a small tank is that they grow at their normal rate until the ammonia becomes so high that they either die or suffer and then can be stunted.  I've heard people say they won't grow to the size of the tank just as a child put in a closet won't stop growing.  And yet, they can't be bigger then the tank.  So, most fish die long before that time.  So, keeping a goldfish in too small a tank will most likely stunt it some and shorten its life.  Keeping koi in a tank under 100 gallons is a recipe for disaster.  Why?  Goldfish can grow to 12 to 14 inches long.  Koi grow to 2 or 3 feet!  So, their body mass it much higher.

Experts say to have at least 10 gallons per goldfish.  For the bigger ones, it's more like twice that.  Experts say to have 1000 gallons for the first koi and 100 gallons for each additional koi minimally.

I've never known a situation with koi kept in aquariums to turn out well.  Baby koi will do fine in an aquarium of 50+ gallons for about a year before they grow too large.

I really suggest getting a 20+ gallon tank and sticking with goldfish.  Save the koi for that day one day when you'll get a pond!  Ponds are so much more exciting!

Good luck!