Why did you treat the tank with Melafix?
Although things may seem fine, 4 clowns in a 29 gal
tank is not a healthy amount. Not just for the potential fighting issue you do not seem to have encountered as yet but also for water quality. The ammonia and nitrite reading you have should speak for itself, both should be undetectable.
Clowns species will live many many years. In fact I am aware of many approaching 15 years or more.
This may not be an eye issue but may actually be a swim bladder/bacterial issue. Can you please give a detailed description of what the eyes look like and how the fish swims when it does try?
How long have you had all the fish and what/when was the last one added?