Emperor angel growth

[ajiisss]

I have an emperor angel in my tank for the last 15 months it hardly grew half an inch . Its around. 5-6 inches . It did start changing colors and is the king in the tank. Its mainly a fowlr nitrates usually under 18ppm feeds 2 times a day tank is 220 gallon . What could be wrong

 

[C_AWOL]

It will start to shoot up in size after it finishes the transition to adult phase IME