I think it is a male. Pics right after a water change can mske it a little trickier. The spotting looks male.....also you said it has a fast growth rate. The shape of the head is looking like a male also. Within the next 1" or so there will be no doubt with good pics. A good amount of coloring around the mouth is a good sign of a male...also the fins will start to color up real soon if it's a male.