The first thing you'll want to rule out is if the tank had suffered from an episode of supersaturation of gas in the water - a sump that evaporates down and "catches air" is one cause, a leaking fitting on the intake side of an external pump is another cause
With that ruled out, at least you can be relatively sure that this problem won't affect any other fish. The issue is that an air bubble beneath the sclera of the eye (without any other signs of swelling, redness, etc.) is almost always caused by trauma. For some reason - certain fish (tangs, butterflyfish and Anthias) are very prone to developing air bubbles following some injury to the eye - most often when the fish is moved in a net, bu also can happen by just running into something in the tank.
The only treatment is TIME. Leave it alone, and some cases will resolve in a week or so. Avoid trying to "de-gas" the bubble with a needle - the extra handling of the fish almost always causes the problem to get worse.
I've had some sucess using a hyperbaric chamber, and pressurizing the fish to around 14 psi in water and oxygen. Home aquarists can't do this as the equipment required (bottled oxygen, regulator and a sturdy pressure tank) is too expensive.
Jay
