They do get tumors Ive heard of it,never seen one.

Hi Mike. Is this lump a whitish gray colour? Sometimes during the winter months, the Koi will get these little greyish white patches and it's normally called carp pox. Not to worry, it's just due to the lack of Vitamin D in your fish due to not enough sunight. In time it can disappear but sometimes it stays.
If you are still concerned, you could remove your Koi from the pond, pat the area dry and add polysporen to the sore and it should start to heal. If it doesn't it could be that water conditions are off. Check your bio-filter to see if it's dirty and make a one third water change from the bottom of the pond.

Your's Koily, Lorraine

Lorraine,
I am happy to see your posts and participation on this forum. That's what makes it enjoyable.
Unfortunately I culled that fish from my pond to avoid any potential problems with my other fish. I don't have a sick tank because it is such a rare event. I learned years ago that doing a wait and see, or treating the hole pond for one bad fish is not wise. This one had a quarter size lump under the skin with scales standing up like dropsy in that location.
Till next chat,
Mike

Hi Mike, I agree with you about medicating the whole pond but sometimes it is necessary. If I were to notice a fish that had a sore on it, I would do what I suggested above. It is stressful on the fish you are trying to heal but not on the other fish in the pond.
The other thing is that this sick fish could be an indicator fish which means that if the immune system on this fish was lowered, (say, due to poor water conditons) it would then give you pause to think: when did I do my last water change, when was the last time I checked the bio-filter, is the filter large enough not so much for the size of the pond but the fish in it, if you have to clean your filter every month by hosing everything down, this means that your filter is not built properly or is only working as a mechanical filter which does nothing to get rid of the toxins in the water.
Also, I used to use test kits on my pond 25 years ago but I found that in some ponds the kits would say that the water was perfect but I would have fish sitting at the bottom of the pond or gasping at the top and in other ponds the kit would say that the water was bad but my fish were swimming around healthy and happy.
Now I let my fish tell me how they are feeling but I make sure that I make those monthly water changes from the bottom of the pond and because I have these great bio-filters, all I have to do is backwash and check to make sure that no sludge is getting behind the window screening to sludge up the scrubbies.

Lorraine