When it is just one fish with just one eye affected, the cause is most likely damage or water conditions. Both eyes, and possibly also with dropsy, is more likely to be infection. Since they are new, it is possible the eye was damaged when they were netted in the shop tank.
For injury, water changes, lots of them. For most damage to fish, lots of clean water is the first thing to try, and it usually works.
And for water conditions, again water changes. Can I just check - did you do a fishless cycle on this tank? What are your ammonia, nitrite and nitrate readings?
Only medicate if things get a lot worse. The trouble with medicating is trying to work out which medication you should be using. With something like popeye there can be several types of infection which could be responsible.