I have a Koi that yesterday looked fine. Today he has noticeable mark on his
forehead that looks like a small crater. Does anyone know if Koi are
susceptible to hole in the head disease? This was the first thing that
popped into my head having had Oscars for some time. This is a rather large
thing though. It looks almost like he ran into something.

My water parameters are very good.

Any help will be much appreciated.

Thanks,

San Diego Joe
4,000 - 5,000 Gallons.
Koi, Goldfish, and RES named Colombo.