Aquarium fish vary wildly in terms of what is normal behavior for a particular species. For a handful of aquarium fish -- and relatively common and popular species -- it is perfectly normal for them to float on their backs. However, for the vast majority of aquarium fish, this is a red flag, a sign that something is seriously wrong and requires attention. Always research any species you keep, so you know what's normal for your pets, and what isn't.
On the freshwater side of the aquarium hobby, the upside-down catfish is known for the way it swims. This species spends 90 percent of its time swimming upside down. Researchers are not certain why it does this, but swimming upside down is normal for this particular species of catfish, and not a sign of ill health. The upside-down catfish is a close relative of the cuckoo catfish, and both species originate in Africa.
Meanwhile, in the saltwater category of the aquarium hobby, lionfish sometimes swim upside down as a part of their normal behavior. Lionfish can manipulate their swim bladders, an organ that helps most bony fish control buoyancy. Lionfish can control their swim bladders to an even greater degree than most fish, allowing them to swim at odd angles, even upside down. So for this species, swimming upside down is well within their normal behavior.
Swim Bladder Disease
Outside of oddball fish like upside-down catfish and lionfish, floating or swimming upside down is usually a cause for concern. This kind of behavior indicates issues with a fish's swim bladder. Damage to the swim bladder is a symptom of a variety of diseases, rather than a specific disease in and of itself. Issues with the swim bladder can stem from shock, physical injury from fighting and infection. If your fish start swimming upside down, you will have to do some detective work to figure out what's causing it, since treatment varies based on the ultimate cause.
Course of Action
If one of your fish starts swimming or floating upside down, look into recent changes to the aquarium. If you've recently added the fish to the aquarium or added water to the tank, shock can cause the fish to swim at odd angles, including upside down. Dimming aquarium and room lights may reduce stress and help new fish adapt. Fighting can also damage a swim bladder. Keep in mind, you may not catch your fish fighting, but if you see bloody scuff marks or ripped fins, your fish may need to be separated to prevent fights. Identify and remove the aggressor and the injured fish will probably heal. Another likely cause of upside-down swimming is infection, which is best treated with antibiotics, available at pet shops.