What fish can swim on back?

There is no known fish species that can swim on its back. Fish are designed to swim with their bellies facing down.

It's possible you're thinking of a specific behavior. Some fish, like seahorses, can swim upright, but they don't actually swim on their backs. They use their dorsal fin to propel themselves vertically.

If you have more information about the fish you're curious about, I might be able to help you further!