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That goes by the common name of popeye. It has a few different causes, but when it is in just one eye, it is due to an injury.
The one eye looks swollen - which may be due to injury/fighting - and if so - will likely heal on its own. However - can you give more information - such as how long you've had the fish - is it fighting/pairing, could the eye have been damaged from net/transfer, etc.
It can also be treated with a broad spectrum antibiotic such as kanamycin, etc in a hospital tank - however remembering that if it's an injury - moving it could be more stressful than watching. Of course flukes are possible for eye issues - did you QT the fish? Have they been treated with Praziquantel?
PS - IF you see marked aggression with the fish separation is best - you can sometimes use an acclimation box if you do not need medicine treatment