It could be a condition known as swim bladder disease. The swim bladder helps fish control their buoyancy and maintain their position in the water.
Swim bladder disease can result from bacterial, viral, or parasitic infections, constipation, and high levels of nitrates and ammonia in the water. It can lead to fish swimming vertically, horizontally, or even upside down.
You should move the affected fish to a quarantine tank to prevent it spreading the disease. Do a water change and get a swim bladder treatment, such as the Interpet one. Also double check you are feeding correctly as per the app recommendations.
