so is the eel all right
gills are usually covered in mucus with flukes
and parts of gill can be eaten away
from here
viewtopic.php?f=25&t=32910&p=361865&hilit=diseases+of+fish#p361865
Gill and Skin Flukes
Flukes are a kind of parasite that attach themselves to specific parts of the fish. Different flukes attach different parts of the body. There is a particular gill fluke that is known to occur only in Discus gills, and is responsible for the death of many juvenile fish. Adult fish can act as carriers and infect newly introduced fish
What it looks like:
Skin and Gill Flukes are usually invisible to the naked eye, but the behaviour of the fish can indicate an infection. Fish that 'flash' or 'strike' objects in the tank may be irritated by skin flukes - those that breathe heavily through one gill whilst clamping the other shut may have gill flukes.
What causes it:
Flukes are parasites that can be carried by Adult fish with little effect. Adult fish can usually suppress any outbeak through their own immune system's defences, however young fish, or uninfected fish that are bought into contact with 'carriers' can be overwhelmed before they develop immunity.
What to do:
Check you water parameters. Treat with Fluke Tabs or Trichlorphon, following the manufactures instructions. Read the warning labels - strong doses of organo-phosphates kill certain fish (like elephant fish) instantly.
There are other treatments, however they involve dangerous chemicals and specialist knowledge, and along with the remedies mentioned above can kill other kinds of fish instantly.