Last summer, I tried to remove the tumor on the fish, but it ended up kind of growing back, but it isn't that big, so I don't think it's too big of a deal.
Koi that are "higher quality" with better size / body shape, color, patterns, etc. are worth a lot more, as they are rarer.
They're sometimes hard to see, but I think they look cool.
I have a goldfish in my pond with a tumor on its back, a neighbor came and left it in my pond when I wasn't home :gig
I paid like $15 for it at a local fish store, LOL