Thank you, Terry.
Now that is a shame that you cannot take a photo, since it would allow me to see just what you are seeing. Still, based on your history of a smooth skin based mass on an otherwise clinically well fish, I have to say that the lesion is highly suspicious of lymphocystis (a herpes virus induced tumor). This virus causes mass type growths on the fish that will appear as soft tissue masses arising from the epidermal layer of the fish (rather then deeper in the muscle). There doesn't tend to be any irritation, ulceration, redness, or infection with these types of lesions. And really they tend to be more unpleasant for us to look at then being actually harmful the fish in itself.
Lymphocystis is actually becoming a more common finding in the domestic goldfish/koi world, mostly due to the ‘silent carrier’ nature of asymptomatic fish. The virus often infects these fish then lies latent, sometimes causing no disease in the carrier fish. Still we tend to see a problem with this virus when it does recrudesce and causes these viral induced masses to appear in the infected fish. This recrudesce is often linked to a weakness in their immune system (either because of their age, stress, if the fish is immunosuppressed or fighting some kind of infection that takes the attention of the immune system). And it is this time of weakness that triggers the herpes virus causes its mischief.
I am afraid that there is no treatment for this virus, and with time they tend to increase in size and can become pedunculated and prone to trauma/infection. Furthermore, while they can be surgically removed by a specialist vet, since it is viral induced there is a high likehood of others forming. As well, I must advise you that if you have had a carrier fish in the tank, he will have exposed all his tank mates. That said, this doesn’t mean we will see this type of tumor formation in all of them (though there is a risk). Their individual risk of development will be linked to potential future immune challenge which may allow the lymphocystis to get ahold.
The only reliable way to get rid of it in the tank would be to cull all fish that are in this tank (which would be unfortunate) and have a full drain/sterilize (probably wait a few weeks) and then start again. Alternatively, you can maintain the stock you have, know this is a risk and make sure water parameters are as best they can be and disease risk factors are as low as possible, to deter lymphocystis recrudensece. I would say if you chose to continue with this stock (I probably would because it would be sad to cull them for a non-fatal condition) that you should avoid bringing in new fish, because while they could theoretically already carry the virus, if introduced to these fish the potential of having it would be more likely, if you know I mean.
I hope this information is helpful.
Please do let me know if you have any further questions.
All the best, Dr. B.