Spasms - convulsions
Fish appears to be having a fit, jerking and twisting abnormally.
Inflamation of the stomach and intestines. Many causes. Very often parasite is causative organism. Pathological examination of a dead specimen should give a pointer as to specific cause.
Untreated or treating without determining precise cause, will usually result in losses. If aquaculturist cannot find to his satisfaction the cause, then an outside laboratory or veterinarian should be consulted.