Any chance of a picture of the fish from the side?
The eye has been damaged and is covered in mucous to help protect it while it heals.
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The normal treatment for damaged eyes is to clean the tank conditions up so the eye can heal.
Wipe the inside of the glass down with a clean sponge.
Do a 75% water change and gravel clean the substrate every day for a week or two.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn/ garden.
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If there's no improvement after a couple of water changes, post some more pictures and add 2 heaped tablespoons of rock salt for every 20 litres of water. Keep the salt in the tank for at least 2 weeks. When you do water changes while using salt, add salt to the new water before it's added to the tank so the salinity (salt level) remains stable.
The salt won't affect the filter bacteria, shrimp, snails, plants or fish.