swim bladder disorder perhaps.
u know like gold fish.
quite rare with polypterus (i think)
possible cause by swallowing bubble of air, bacteria, floating food, etc.
to cure it, stop feeding your poly i guess. keep change the water about 50%-80% volume will help. need lots of time to recover though.
is your poly had poop lately? if it hadn't your in trouble.
the pressure could break the swim bladder. if that happen, well u know what gonna happen.
if it gold fish, i would say try feeding it with peas. but its a poly.
sorry mate...