There are tench in my locale that have a genetic red spots and a stripe or two. They are perfectly healthy. In fact I saw one in Nth Yorks like that last year which had been originally stocked from Lincolnshire.
The one in the picture just might be a more pronounced colour variant of this type and not diseased at all? The colour of the red appears similar. I did hear once it was something to do with the colours that cause those golden tench.