Rosy Barb Has Swollen Eye

[vinodjbhatia]

Hi

Tank params -

30 gallon, 2 filters - one a JBL Cystal profi e900 canister with 900 lph output and the other a top filter rated at 400lph.
Tankmates - 6 serpae tetras, 4 rosy barbs, 4 dwarf rainbow fish, 1 dwarf gourami, 3 pepper cories, 1 rainbow shark and 1 BN pleco (its overloaded - my first tank, but i am not adding anymore)
Moderately planted, fine gravel as substrate.
Temp is between 25-27C
Ammonia, nitrite - 0
Nitrates - currently below 5. I did have a nitrate spike some time back and despite my best efforts to cut it down through larger water changes, I could not control it. Finally I started using Seachem Purigen and that has helped.

Now the problem -

One of my rosy barbs is having a swollen eye and I do not know the reason. Pics are attached. The fish is behaving normally and apart from the swollen eye, there seems to be no other problem. The other fish in the tank seem fine.

Is this a disease or an injury and how do I treat it?