Why do normal fish get dwarfed

[Opcn]

I've got a friend with 5 indo pacific blue tangs that only got to be 6-7 inches long they seem healthy and are 3-4 years old but they are just small. How does this happen? are the fish unhealthy? they come from two different sorces (aquarium dealer and internet) and theyhave normal colors but they just ended up small. They are in a 150 gallon speceies only tank if you care to know.

 

[Eelzor]

The size seems big, but fish have a hormone that tell how big the environment they live in is, so they simply stop growing if they feel crowded. It would be my only guess.

 

[Opcn]

They are supposed to grow to 12 inches.

At this point if I buy some and move them to a bigger tank will they grow more or are they stuck at this size forever?

Thanx

Opcn