Cant see any pic but he red X
First step is a water change, about 30 percent. Get one ready and let it mix for 24 hours with an air stone or other method of heavy aeriation.
Next some water paramiter if you please. Tell us about your filtration. How long the fish has been in the main display? Has a net been used on it latly, any aggression with this fish vs. the other fish.
It is normally caused by environmental conditions like poor water quality, high DOC’s, high nitrates, improper catching like when using a net, fighting with other fish, or by having hit something hard in the tank like rocks, heaters, or any other stationary object. It can also be caused by bacteria, parasites, virus or a growth behind the eye.
So let us know what your water paramiters are..ammo, trites,0 Nitrate, phosphate, temp, salinity...
Also do you have a qt tank set up?
Thomas