Aquarium fish happily swim and hang out at different depths of the aquarium. Some fish varieties prefer to dwell mostly near the bottom, while others are more inclined to spend their days in the middle or upper regions of the aquarium. One thing you don’t want to see is a fish that is spending a lot of time right near the surface of the water—perhaps gasping—and in apparent distress.

See Full Article: https://www.aquatic-eden.com/why-do-fish-swim-at-the-top-of-the-tank/