Was winterizing the koi fun they realized this guy was sick please help!

[Hellothere116]

Have you tested your water?

Yes

If yes, what is your ammonia?

.0

If yes, what is your nitrite?

.10

If yes, what is your nitrate?

0

If I did not test my water...

...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

What percentage of water do you change?

21-30%

How frequently do you change your water?

Every month

If I do not change my water...

...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

Any idea what it can be? He's the only one with it?
Just took him down stairs thinking about treating with Seachem KanaPlex?

 

[Hellothere116]

Nitrite is 0 nitrate is .10!

 

[kendragon]

Carp pox