I have a Pygmy halfbeak that seem to have developed a bent back. It swims with an exagerated wobble and it doesn't seem to be able to stay level -- eventually it takes on a headstander pose.

Its appetite is still undiminished, but its ability to compete for food is greatly reduced by its disability. It loses in all its fights with the other halfbeaks too, so it's now missing some of its tail, which exacerbates its condition.

What can be the cause? And what can I do for it?