Thank you for the additional information.

Sadly, it appears you didn't post your photos, and I must say that I do feel that they always help because they let me see what you are seeing in their fish. As well, your treatment regime is both very impressive and commendable.

Anyway, based on your history, treatments, and the description of the lumps that are affecting multiple fish, I'd be highly suspicious that you are seeing the work of lymphocystis (a herpes virus). 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.

As well, I must advise you that if you have had a carrier fish in the pond, he will have exposed all his tank mates. This explains why others have the masses but even those that don't are now going to be carriers of the virus. 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 pond would be to cull all fish that are in this pond (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 I am sucker for broken animals) that you should perhaps 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.
If you have no further questions, feedback is always appreciated.

All the best,

Dr. B.

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Thank you for the photo.
It does look very much like lymphocystis here.
I am sure you can appreciate how it is arising from the epidermal layer rather then pushing up from underneath or being a cyst on top of tissue. It is quite characteristic of the virus.

Dr. B.

Thank you for the update.
That sounds like a good plan at this stage.

Take care,
Dr. B.