Goldfish has red sores/lumps on back

Hello, I've had my white goldfish for about 3 months, they are in a 100l tank and when they get bigger I plan to upgrade it. At the moment they are very small and recently I added more goldfish, a few days later the white goldfish has red lumps on it's back. The white goldfish has always been sitting in the corner of the tank ever since I got it but does eat and poop normally. Is it a disease passed on from the new goldfish or is it something else? Thanks

it looks like someone has pulled anchorworm out of the fish and the red spots are the sores left over.

do a 75% water change and gravel clean the substrate. make sure new water is free of chlorine/ chloramine before it's added to the tank.

add 4 heaped tablespoons of rock salt, sea salt or swimming pool salt, per 20 litres (5 gallons) of tank water. keep the salt in there for 2 weeks and see how they go.