In the first place, “fishes” is the correct form for the plural of “fish”.
Second, and to answer the question, fishes don’t “breathe” oxygen, although like most fauna (not all) they require oxygen to live. They extract it with their gills, where their blood extracts it from the water directly. Almost all water (fresh and salt water) contains dissolved oxygen in some concentration. In fact, because of the extreme high pressure of deep water, the oxygen may be even more dense at some deep depths than it is closer to the surface. (I don’t know this for sure, and I could be wrong. I don’t study the topic.)
But fishes do not “breath” (respire) in the way that sea mammals such as whales and dolphins do, or amphibious animals such as turtles and crocodiles. They use their gills, and they do not rely upon “air” to breathe.
Lobsters and scallops, for example (and shrimp, too, and many varieties of clams) never come to the surface for air. They all have respiration mechanisms that extract the oxygen they need directly from the water.