Fin rot in a koi. My boyfriend have had this koi for 8 years now. It suddenly go

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Fin rot in a koi. My boyfriend have had this koi for 8 years now. It suddenly got problems eating and shaking its head. We found a big rock in its mouth that was stuck. We tried to be gentle when removing the rock but it ofcourse caused a lot of stress to the koi. 4 days later it has got fin rot. Active and eating well, normal behavior. It lives in a garden pound, 200 000l, good water quality. The other koi are healthy. Can it be better without treatment since the stress factor is removed, or do we have to move it to a tank and examine it more and treat it (Nifurpirinol/Metronidazol)? I will be very thankful for any help and advice