White koi with reddish skin on rear end

[LOCOSPOTTER]

I have had this white koi developed red blotchy patches on the rear end it has been going on for months coming and going. I have given it two salt baths six weeks apart the last one today. Three years ago it lost all of its dorsal fin very quickly but it is feeding and swimming OK but I would like an opinion on what is the problem is so I can treat it,I am thinking it might be a bacterial infection. It's a 2ft fish so I don't have a hospital tank to keep it in unfortunately only a big blue treatment bowl. Any suggestions would be great help.

[Maddog1]

Hi/ How's your ammonia readings?

[LOCOSPOTTER]

Hi/ How's your ammonia readings?

Ammonia levels 0 yellow on test tube .

[Alburglar]

Assuming it's not irritated due to parasites/ ph/ chlorine

Tamodine and keep an eye on it.
Usually nothing to worry about.