I have mine for over a year now and they haven't grown much, 1/2" max, the biggest almost reaching 4" in an approximately 100g with 50-60% waterchanges each week. So they seem to be very slow growers or just don't get that big. One of the most beautiful Metynnis sp in my eyes though, especially when the males start chasing after the females. They even get black stripes at the tip of the red fins.
