growth rate differs from fish to fish.
My male was only small (about 3") when I got him and now 4-5 months down the road he is about 6", where as the female I bought a few months back was 4" (the male was this size at the time of purchasing the female) is now 6.5 - 7"
Make sure you feed them high protien foods and they will grow quicker than if fed food with low levels of protien, as remember these are carnivores.
