Whats causing this eye issue

[kiwis]

I've got a couple of yellowtail damsels in my tank, some of my starter fish.

Today I noticed one staying rather idle in the back corner, on closer inspection he's got a bung eye... he's not turning around for me so this is the best photo I can get.

Most worried I don't need to treat anything else in the tank for any disease.

 

[MombasaLionfish]

That is popeye I have heard epsom salt helps but do not take my word on it. Did you QT them?

 

[rkpetersen]

Single eye popeye can be due to injury or infection. If it's both eyes, it's usually infection.
If you want to treat for possible infection, you'd want to remove it to a quarantine tank and add Metroplex, Kanaplex and Furan-2 to the water.
Epsom salt (pure magnesium sulfate) can be added to tank water at 1 tablespoon per 5 gallons. It may or may not help reduce swelling.