Red cysts on butterfly tails

[milehighkoi]

Hi Koiphen,

I've been chasing these lumps that develop on my koi's tails for months. They last a long time, at least multiple weeks, and only seem to appear on butterfly fins, typically near the base of the tail. Initially, I thought they were juvenile anchor worms, so I treated the pond for those. However, I've never found an adult anchor worm, and despite a few rounds of Dimilin X I still find these lumps, so I guess it must be something else. Any ideas?

Attached are some photos, some attained after significant struggle. If there's an etiquette for number of photos that I'm violating, I apologize!

[RichToyBox]

I would still say it was encysted juvenile anchor worm and due to the colder temperatures, it may take a couple of more months for them to develop enough to come out of the encysted state to a place that will allow the treatment to kill them. Continue the Dimilin through spring.