Question

koi   hi one of my large chargoi has a large lump on its side which oi have no idea what it is water quality is good and feeding with no problems can you help many thanks

Answer
Dear Michael,

It looks like an ulcer or maybe some extended growth of flesh after A wound or parasite infection.
If you like A experienced vet can take this of by using nitrogen or by surgery and sent some sample to the lab to make sure.
I think this is the best way to go to be sure.
Maybe first you can check if there are wounds visual and take A scrape to check for parasites.
After this you can act by doing woundhealing (products available at your expert koi stores).

Good luck!
Kind regards,
Raf