It will grow fast till it gets up to 15" and the growth rate slows down. Mine is at 18" right now and doesn't seems to grow anymore. Was hoping it will get up to 20"+. It still got it's external gills though.
That is one slow growing weeksii. At 10" and with a year in only grew by 2". You must be doing something wrong. Lack of high protein foods and regular water change...?