Tetra with large lump on left side !

[Sebsfish]

A lump showed up on my tetra about 7 days ago. It rapidly grew to the current size. It is on his left side only and he is acting normal. No other symptoms. Two other tetras with him. They are fine.
Water levels are good and so is temp.
They are two years old.
can anyone help me? I’m in charge of their care while my son is away in the military.

 

[Aiken Drum]

Could be dropsy? Sorry, but im not great at diagnosis and treatment. Can you isolate the fish? Dropsy is normally not treatable.