^ this.
I am only a human doc, and not a vet. But to swim like that you need a loss of orientation. Either at the source (balance/input, swim bladder etc) the pathway (nerves/infection/injury), or the controller (brain/infection/injury). In humans a simple virus can cause it, and that can be temporary. Or, cancer. A course of antibiotics might not hurt, I would definitely move him to the sick tank. At the minimum, the other fish might take the opportunity to beat him up.