That’s a bit different for tropical fish. While salmon live in cool water, which typically has lots of dissolved oxygen, tropical fish are more likely to find themselves in oxygen-poor environments. As a result, they’ve developed several special adaptations to being short of breath. Some species, including many of those sold for home aquariums, will engage in “aquatic surface respiration.” They swim upward for swigs from the top-most layer of water, which has been exposed to the air and therefore contains more oxygen. Others—including sculpins and gunnels—pop out of the water for gulps of air. And then there are those, like the lung fish, who use internal organs as makeshift lungs, absorbing oxygen into the blood through the walls of their mouth, swim bladder, or even stomach.