Sounds like a buoyancy issue.
What antibiotics are you using ?
Struggling to maintain a normal swimming position is relevant to swim bladder infection or similar iternal issue/symptom.
If severe buoyancy problems exist, the fish may not be able to feed normally or even reach the surface of the water. This disorder is sometimes caused by compression of the swim bladder, which may involve a distended stomach from rapidly eating, overeating, constipation, or gulping air, which is thought to occur with floating foods. Eating freeze-dried or dry flake food that expands when it becomes wet can also lead to an enlarged stomach or intestinal tract.
Other causes are :
- Trauma/Injury
- Bacterial infection
- Parasitic infection
- Stress/rapidly changing water parameters
- Poor water quality
Water change(s) may help and antibiotic should also help. Kanamyacin or neomycin should help great and best done in a quarantine tank