glumshoe:

Fish will “grow with the size of the tank” in the same way that your feet will grow with the size of your shoes. 

Fish in too-small tanks will stunt. Their bones and musculature stop growing, but their internal organs keep growing. You can tell when a fish has been in a too-small tank for too long because its stomach and eyes will be bulging and the eyes will be too large. This will eventually kill them as their organs crush against each other.