The new pics are much better! But it is difficult to make an exact diagnosis based just on the bump, but I think we can narrow it down a bit.
1. The raised scales would indicate an abnormal & fast growing mass - but we know that already.
2. The location is over the thorax - so it is not likely to be things in the abdomen - so it is prob not stool impaction, ascites or dropsy.
3. Soft & mushy lumps are usually not tumors - they should be firm & fleshy.
4. The lump is likely in the muscles or just under the skin - as there isn't any organs under that area except the heart.
My guess is that it is either an abscess or a parasite. Possibly both - eg. a fluke or worm entry site that got infected. It is more likely an abscess if the lump felt fluctuant (ie feel like a bag of water that is squishy in additional to being mushy ....)
I am not sure what to do with this, without a firm diagnosis. A vet would do skin scrapes to look for microscopic parasites, & perhaps drain the abscess if it indeed is one. My gut feeling is to do nothing (just yet) unless the fish is showing other signs - such a lethargy, flashing, etc - except providing clean water. You might consider isolating the fish in case that is parasitic. <If it is a parasite like a fluke, when that thing bursts, there will be lots of little parasites released.> Give it a few days & see how things pan out.
I hate to dump in meds blindly, but if the fish is also acting sick, I would try a broad spectrum antibiotic and an anti-parasitic med. In goldfish, you are best to use injectable or medicated foods. Pretty much anything you can put in the water is not efffective. The current best choice is kanamycin for antibiotics. <Note that antibiotics are not that effective in an abscess until it drains - and it will usu. do so on its own> For parasites, something with Plaziquantil &/or metronidazole (Flagyl).
This is the info on medicated foods in case you need it:
The Goldfish and Aquarium Board (GAB) Articles - Medicated Food Explained and Explored
