I assume you mean the lump is large enough to show on both sides of the body, and it is the one lump?
I know golfdish can get carp pox, which is like the wart virus in humans. This forms white lumps on the fish. Looks ugly but is harmless. Don't know if other fish can get this.
In the tetra's case, I would wonder about it having had a foreign body stuck in the scales, which has got infected, or maybe a cancerous type tumour. Fish do get these and they are incurable. Doesn't spread to other fish either.