We all know fish can't breathe, or well, at least not in the same way we do.
Some fish out there don't need lungs to breathe air. One fish family, the anabantids, has evolved a special structure called a "labyrinth organ" that actually lets them breathe air through their heads! Although the labyrinth organ isn't a real lung like humans have, it's a pretty complicated organ. It's really just a backup for the fish's gills.
Why would fish develop something that complicated just as a backup?
Well, it gave these fishes' ancestors a big advantage. These types of fish all developed in areas where the water is warm, shallow, and stagnant. Warm, stagnant water doesn't hold much oxygen. In that environment, a fish that could get extra oxygen from the air would have a big advantage over fish that could only breathe in the water!
Some of these fish can even live out of water for a few hours. One famous anabantid, the climbing perch, can crawl across land to a new pool if its home dries up, but it has to stay moist to survive, so it can't live out of water too long!
Some popular aquarium fish are anabantids, like gouramis and Siamese fighting fish. They do well in aquariums and bowls because they can thrive in stale water, as long as they can get some fresh air!