eye issue

Tank size: 180L (101cm in length) tank age: 4 years pH: 6.8 ammonia: 0 nitrite: 0 nitrate: 0 kH: 6° gH: >7° tank temp: 27°C Fish Symptoms: Swollen eye for about a month now. It suddenly appeared one day, which led me to believe he hit something and the swelling was caused by trauma. After it...