Sounds to me like swim bladder disease or infection.which is common with this type of fish. Some signs of swim bladder disease is a fish that is unable to swim normally. The fish looks like it is having problems staying balanced or upright in the water. Sometimes, fish with swim bladder disease seem to bob back and forth ir even backwards in the water, darting down and then bobbing back up. Other times they may just look like they’re laboring or swimming really hard just to stay in place.
Some typical causes are injury or trauma, poor water quality, parasitical or bacterial infection, and sudden changes in parameters
If fish is eating, try kanamyacin with focus as a binder to the food