Is it just me, or is there like, three bubbles on the bottom fin? There seems to be some discoloration there too. His tail fin looks kinda tatterred as well. Was he like that when you got him? Which store did you get him from?
Here's some info I found online:
Gas Bubble Disease
Gas bubble disease is not caused merely by air bubbles in the water, as the name might lead you to believe. It is caused by oversaturation of dissolved nitrogen gas in the water. Standard airstones and water changes don't harm fish by putting bubbles in the water. Pump, fountain or filter failure, increased temperature or excessive aeration, however, could result in this sometimes fatal occurance.
It is similar to when a scuba diver rises up from deep water too fast, and the change in pressure causes the "bends." The water (in this example, that which is within the bloodstream) is forced to contain more gas than it can normally hold, which causes supersaturation. When this difference between gas pressures occurs, the gas seperates too quickly out of the bloodstream, leaving gas bubbles behind.
Symptoms:
Fish affected by gas bubble disease may have visible bubbles under their skin/scales, or in their eyes. Or they may simply act sick if the problem is internal where you can't see it.
Solutions:
Stirring up the water may help hasten the degassing of the water a bit, but it doesn't do much for your fish at all. Examine equipment: if you have a submersible pump, be sure it's not sucking in air: be sure it's covered sufficiently with water. Equipment that is pulling air in with the water will cause problems.