Well, as you can plainly see, they can get downright huge! Please do remember this photo when you're trying to stick a 4" little koi yearling (tosai) into a 350 gallon pre-formed pond. Koi can quickly outgrow small ponds, which often leads to death or an unpleasant situation.

If you've got a small pond, stick to goldfish--as they do better in smaller quarters and will restrain their growth to the size of the container it is in. Koi can reach 24 inches or more at maturity. Heck, by their second summer they can easily be at 10-12 inches! Realistically, a pond of less than 1,500 US gallons is not really suitable to house koi. But, If you're insistent on it however, at this size stick to no more than 2 or 3 (considering for their future size), and make sure your filtration is maxed out.

So, when you see that cute little baby koi in the shop, please do think about how big it will ultimately get. After all, you wouldn't buy a 2 foot tall dog house for your Great Dane puppy, would you? It's no different with koi.

photo by wefish.de on flickr