if you put a person in a room that was 4 feet tall when they were 6 years old and left them in there until they were 23, they would be less than 4 feet tall. its called stunting because it stops growth, and prevents an animals or person from getting to their predicted height/size. the same goes for fish. it is caused by overstocking and/or a tank that is to small. it also has to do with the growth hormones that the fish releases into the water. to much of it and your fish wont grow as much. when a fish is stunted, its insides keep growing while its outsides stop.
if you dont think stunting poses a threat to a fish health, just read my story. a couple of months ago, i recieved an angelfish from my aunt. she said that she rescued it from her neighbor that was moving and was going to flush it before she moved. i asked what size of tank it was in, and my aunt said the lady had it in a 5 gallon tank for about 2 years! in that time, this fish shouldve grown to be at least 3.5 to 4 inches on a basic diet, but it was only about 1.5 inches long. there is no question it is stunted. how do i know? i have 6 other angels that range from 4-6 inches long. its a shame too because this angel has some of the best colors i have ever seen (i posted pick a few days ago). moral of the stroy-dont buy fish that get to big for your tank and if they do, get them a bigger tank. DONT STUNT YOUR FISH!