gold fish with lump

[jstephan]

I have a large fantail who has a lump on the side towards his tail. When you look at him swimming it is hard to notice - but when I feed him in the morning & I am looking down on him you can see it well. The skin is not missing and there is no scale damage - almost like a lump under the skin. The aquarium guys suggested a tumor but I have also within the last 6 months lost another large fish who had the same thing. Can anyone offer any suggestions!

[VoidParadigm]

Sometimes fancy goldfish are bred poorly, causing it to grow deformed internally oddly.

I think it'd be a tumor, though. Nothing you can do except see if the fish recovers, I think. If it gets too bad looking (ie. starts hindering swimming or looks painful due to it growing) you may want to consider euthanizing. Possibly also just a cyst, which could clear up on it's own, or be there it's whole life without issue.

What size tank is the fish in?
Sometimes a tank that's too small can cause stress, which can lead to medical problems.