What is this ............

[Carz]

He has a patch on both sides of him. Any idea what it is?

 

[Matt L]

#fishmedic

 

[MnFish1]

This seems to me like a significant bacterial injury - probably systemic - that will be difficult to survive.

More history is essential

 

[i cant think]

He has a patch on both sides of him. Any idea what it is?

Looks to me like an infected ulcer.
This is an Ulcer I had on my Angel when he was male (He did heal, I used melafix for a couple of days until the main damage was gone but the rest just naturally healed). The reason I say it looks infected is usually a fresh/uninfected ulcer is Red similar to the photo below.

 

[monkeyCmonkeyDo]

Possibly ammonia burn?! I had a silver side mono that had a patch like that. He recovered. I also agree could be a bacterial skin infection. Either way my silver mono recovered. Hope your guy pulls through.
D

 

[nereefpat]

If possible, I would get the fish into a cycled QT and treat with antibiotics.

 

[Spare time]

I'd definitely try to treat this as soon as possible.