It looks like his eye was injured and then infected... I would do as Sam said, then add salt, 1 tbsp per every two gallons, over a period of 2 days, giving a dose every 8-12 hours. Then, keep the temp at 78-80 F, then see how he is doing 2 days after the last dose of salt. Say the tank was a 10 gallon hospital tank. You would add 1 tbsp of salt every 10 hours until you have added 5 tbsps all together, then see how he is doing in 2 days, feed only once or twice a day. If his eye is somewhat or noticeably better, leave him in the tank. If he is not better at all, add 1 tea spoon of salt per every 2 gallons all at once. After two more days, he should be better.
