Most fish in a pet store will not be stunted in growth, they are held in huge storage vats when at the wholesellers. Once they get to the petstore they are rarely there long enough to really stunt the growth. As for Koi, they are going to grow no matter what, even if there is some stunting they are still going to get very big. koi do grow a lot in the first year, they can go from fry to 6-8" in the first year.(in optimal conditions) In the pet stores they may not grow that fast, they don't get fed like they should and limited space will reflect a slower growth rate, however this does not mean they are stunted, it simply means they are growing bigger slower. It's when A fish gets to a certain size and has NO room to grow that stunting starts really taking place. Example, keeping a 10" koi in a 10 gallon tank for years. If it's only allowed to reach that size and not given any new room to grow is where stunt issues come in, They can still grow if given a new tank or pond to grow in but by that point they may not grow to their full potential. Keep in mind though that Koi can get to 36" even possibilities in some to reach 42" That's a huge fish that needs lots of swimming room, and very good water quality. 1 fish that size in a 1000 gallon pond is about the same as a 10" fish in a 10 gallon tank, Water quality must be kept up with! They poop ALOT!