More questions.
Is this the only fish exhibiting symptomatic behavior?
Do you have other photos of this particular fish?
It appears that the fish has 'clamped fins', is this so?
Do you have the means to quarantine this fish for several days?
At his point the only thing that I see causing a problem is the Nitrite level. This should be 0.00. This may be caused by too little available biofiltration for the fish load/density in your pond.