From my experience they grow fast up until around 6 inches then slower after that. Females will almost stop once the spawn for the first time. If you have a dominant fish (especially a dominant GT) that is a lot bigger than a newer GT introduced they seem to grow really slow. I am having that in my tank right now. My female rules the tank and she is about 6 inches or so. The mail is about 3 inches and is growing slow as cement.