Fishes are great swimmers, and this ability comes not from practice but from anatomy. Fishes have a slender body which they flex back and forth to cut through the water, and several fins that help them move, turn, stay upright, stop and so on. In addition, most fishes have an internal air sac called the swim bladder that allows them to control their buoyancy and orientation without having to continuously swim and thus expend energy. When fishes want to stay afloat, they gulp in air and inflate the bladder and when they want to sink they deflate it.
The swim bladder is located near the bellies, below their center of mass, which makes them prone to rolling. The fish tries to negate this effect by flapping its fins. But when the fish dies, it lose the ability to maintain equilibrium and the more buoyant part of their body tries to float to the surface, flipping their bodies upside down. A fish swimming on its side or floating with its belly up is a sure sign of injury or death, but for a few remarkable fishes, being upside down means everything is great.
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An upside-down catfish. Photo: Ricky Romero/Flickr
Source: amusingplanet.com