Hi and welcome to the forum
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the water before adding it to the tank?
How often do you clean the filter and how do you clean it?
How long have you had the injured fish for?
Have you introduced any new fish or plants in the last 2 weeks?
Have you check the water quality for ammonia, nitrite, nitrate & pH?
If yes, what are the results in numbers?
Please post a picture of the injured fish and tell us what other fish are in the tank.
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Most eye injuries heal up quickly in a clean aquarium and don't require medication.
Wipe the inside of the glass. Do a 75% water change and gravel clean the substrate each day for a week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If the tank has been running for more than 2 months, clean the filter if it hasn't been done in the last 2 weeks.
The following link has some information about what to do if your fish get sick. it is pretty long and boring but worth knowing. I recommend printing it out and reading it in bed to fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/#post-3804819