I always thought cloudy eye/popeye was related to water quality? However treating a fish with antibiotics in a smaller tank is always a crapshoot (because of water quality problems) and every time you do a water change you are decreasing the amount of antibiotic, rendering it pretty much ineffective.
IMO, I would have just made sure the fish was eating and left him in the main tank and done a few water changes. I'm not sure if continuing with erythromycin at this point will do much for your fish. But I'm not sure, maybe someone else will have another product to recommend (although I'm pretty sure this is the right antibiotic to use for this condition).
Also, there are products you can get to reduce ammonia in the tank (ammo lock, Amquel) which might help without having to do waterchanges. Hopefully that will help
