I was also surprised about the O2 thing. What happens is that in conditions of bright sunlight, high nutrient levels and warm water, algae and plants produce O2 faster than it can diffuse out of the water at the surface. This creates a high partial pressure of O2 in the water, and also in the tissues of any fish in the water. When the light level is reduced, O2 production is reduced and the excess O2 in the water diffuses out at the surface. This leaves the fish with a higher partial pressure of O2 in its body than in its surroundings. Most of this is used for respiration or lost through the gills, but in tissues with poor circulation and low O2 consumption (fins, eyes, skin) it comes out of solution, forming bubbles. These are soon absorbed, and the tissue damage usually heals fast.
Absorption of O2 is probably mostly through the gills, but the fins, eyes and skin surface may absorb from the water directly.
I am not sure about if it is more likely in fish which are adapted to low O2 conditions. Certainly the goldfish has very high tolerance to low O2 - this is one of the reasons it is such a popular pet. Few other species would survive in a bowl or unfiltered tank (although they are probably not happy in those conditions).
If fish are kept in water which contains nitrogen under pressure (from a well or borehole, or malfunctioning pump), this can cause nitrogen bubbles, which are far more serious as N2 is not used by the tissues. It could probably be treated in a decompression chamber, but I haven't heard of that yet.
