xanthic fire eel problem

[PartyHacks]

Have you tested your water?

Yes

If I did not test my water...

...I recognize that I will likely be asked to do a test, and that water tests are critical for solving freshwater health problems.

Do you do water changes?

Yes

If I do not change my water...

...I recognize that I will likely be recommended to do a water change, and water changes are critical for preventing future freshwater health problems.

What is this? this has affected 2/3 of my fire eels.

How can i fix? doing frequent water changes 25% a day and all other fish are fine. levels are all within normal ranges.

Water looks like it does because I scrubbed algae and noticed and took a pic right after.

What treatment would the veterans recommend?