Why Do Fish Swim Upside Down?

A little catfish swims belly-up in an African river. It's perfectly healthy. A goldfish tips over in a home aquarium and struggles to get righted. It's not healthy at all.

Two completely different stories in the same position. Then, why do fish swim upside down? The short answer is that a few fish are built for it, but for most pet fish, it is a sign of trouble. Let's consider both sides.

Water makes fish almost weightless, so why do nearly all fish remain upright?

Now the strange thing is, the majority of fish are top heavy. Gravity actually wants to roll them over, just as a kid would struggle to stand on a beach ball in a swimming pool. Fish keep themselves upright all the time.

The reason, scientists think, has to do with how fish bodies first evolved. The brain, nerve cord and eyes ended up on the upper side. The mouth and stomach ended up underneath. After that body plan was set, there was seldom a good reason to alter it.

Staying upright also brings real benefits:

There are exceptions to the rule with some fish. A group of African catfish spend much of their lives belly-up.

These catfish come from bottom-dwelling ancestors. Their mouths are on the underside of their bodies. The surface water has a higher oxygen concentration. Turning over let them use their downward-facing mouths at the surface. As one McGill University researcher put it, to these fish the water's surface is "just another substrate."

Some reef fish also turn their bellies toward walls or overhangs, then flip upright again in open water.

Most aquarium fish, such as goldfish, bettas and guppies, are not built to live upside down. If your fish keeps flipping over, it usually points to a buoyancy problem. Vets call this a swim bladder disorder.

Inside the fish is a gas-filled organ called the swim bladder. It is similar to an in-built float. By controlling the gas, the fish can rise, sink or hover without effort.

When this organ is pressed, damaged or not working well, the fish loses control. It may float, sink or roll over. Important point: a swim bladder disorder is not a disease on its own but a symptom. It's caused by something else.

Vets list several possible causes:

Swimming upside down often comes with other signs:

Does a fish that is swimming upside down mean that it is going to die? Not always. When the cause is diet or water, the outlook is often good if it is corrected quickly. In cases of long-term infection, tumours or organ damage, it is more serious.

Thus, why do fish swim upside down? Some, such as certain African catfish, do it by design. However, for most pet fish, flipping over means that the swim bladder is not working right. Test the water, move quickly and get a vet involved. That gives your fish the best chance of swimming upright again.