how long has it been there for? if you just noticed it now, i could be a natural reaction due to intense fighting with other tankmates. if its been present for a day or more, treat it for Popeye. Popeye medicine is available at LFSes.
I just noticed it today. I looks as if there is a bubble or blister on his eye, it could be from fighting with tank mates because I saw that the other pair of cons has eggs on there side of the tank. I will go get the popeye treatment just incase. Do I need to treat him in a different tank? or is it ok to treat him in the tank he is in with the other cons.
do not treat your main tank , most meds wipe out your biological filtration (good bacteria)
treat him in a hospital tank. Also I have had better luck treating popeye with water changes. What are your water parameters? Change your filter cartridge too.