How long has it had the swollen eye?
How long have you had the fish?
How long has the tank been set up for?
How often do you do water changes and gravel clean the substrate?
How often do you clean the filter and how do you clean it?
What other fish are in the tank?
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The best thing to do if your fish get sick is a 75% water change and gravel clean the substrate every day for at least a week or until the problem is identified. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Wipe the inside of the glass down with a clean fish sponge.
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 them.
The following link has information on what to do if your fish get sick. It's long and boring but worth a read when you have some spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...