What wrong with my FH ?

[Costrines]

Hi everyone, please help me to identify which illness of my FH. He become shy and try to hide behind the leaf, filter pipe. Slow eating & black scales appeared in many spot. I thought he have too much stress but not so sure. Water parameter :Nitrate 18mg/l , Nitrite 0.5mg /l, 14dGH, 10dKH, PH 7.2 . Pics below.
Thank a lot

 

[Man-made Monsters]

Nitrite at .5mg is the problem. More water changes. I would start doing 20% every other day. Flowerhorns are very sensitive to nitrite.

 

[Ruturaj]

I would also use some binder to detoxify Nitrites with water changes. What's ammonia level at? Keep lights low, feed lightly till he is all okay.