whats this? (not for the squeamish)

[Tomfish]

Poor chap, imagine that in your face

Its curiosity really (raised by an earlier post), it only affected a handful of fish and this was the most unlucky one, then went way after a few weeks. They seemed to be parasitic worms, but I was not able to find anything that matched online. The nearest thing was anchor worms, but these seem more burrowed in. Any ideas?

 

[Henry]

Gill flukes? It's hard to make out from the picture. I'd isolate and treat the affected fish asap. In the mean time, keep the water well oxygenated.