They shouldn't really have a light on the floor of the tank, and if the 'xterra 10' you're referring to is the reptiglo, that could be causing eye issues, especially if they can get that close to it. Ideally they want to be about 6-8 inches from their UV, the amount of UV rays they get gets exponentially larger the closer they get to the light, which is why having one on the floor isn't a great idea, and the reptiglo bulbs emit too many UV rays at a certain frequency which can damage their eyes. Can you remove the one from the bottom and mount the top one underneath the screen lid so he can get within the 6-8 inches while basking? This would certainly be a more normal set up and rule out the possibility that the UV is the culprit.
However it does seem rather strange that the swelling is only in one eye and responds to antibiotics. Either way, I'd still make the changes suggested above and see if it helps him.