Parrot Fish Stops Growing

[coolfishfan]

I had bought a red parrot fish last year and kept it for 6 months.
I took proper care of it and it was growing up very well.
 
A few months ago my friend burrowed it from me and kept it for 3-4 months.
He returned it 2 months back. He apologized for not being able to take proper care of the fish.
 
Now I can notice that the parrot fish is fairly normal but has STOPPED GROWING !!!
From a couple of months I am taking proper care of it but NO IMPROVEMENT.
How can I RESUME its GROWTH........?

 

[fluttermoth]

How big is the tank, and how often and how large are your water changes?

 

[coolfishfan]

The tank size if 36*18*18 inches and just four parrot fishes in it.
I change 20% of water every week.
 
I don't think there is any problem with the water conditions.
The rest of 3 parrots are in excellent health and growing very well.
 
The one that I mentioned also looks pretty healthy and active.
The only thing is I want it to grow like the rest of 3.

 

[Lunar Jetman]

How big is it then? And how much smaller than the others is it?

 

[coolfishfan]

3 of the parrots are 3 inches and one is 2 inches.
 
all of them were of the same size when they were kept together.