Bubbles/blisters can be found beneath the skin and inside the body. They are mostly found around the head and the eyes.
Cause:
A condition known as "over saturation" is created due to the dissolving of excess amounts of gas. A certain amount of gas is always dissolved in liquid in relationship to such factors as pressure and temperature. When these gas levels are to high, the water will constantly attempt to release the gas in the form of small bubbles. Too much sun and the heavy plant and algae growth associated with this is a common cause of over saturation. Since the plants take up and release a lot of oxygen, the fishes' blood can itself become over saturated.
Treatment:
Reduce the amount of sun light the aquarium is exposed to. Further, an air stone can help prevent problems.
