lump on phantom

my phantom tetra has a lump where the tail meets the body, also the tailfin is looking pale, there are no signs of rot or fungus, and the fish is eating and swimming well and is shoaling with the others that all look fine.
i have put it in a hopital tank with a cycled filter. would aquarium salt help, or is something stronger needed.

125 litre tank
no new fish added
ammonia 0
nitrite 0
nitrate 5
ph 7.5

any help would be really welcome, thanks chris.

thank you for interest, my camara died before i got a chance to get a good photo. hopefully upped a pic from daughters phone(mine only makes phone calls). will upload abetter photo in AM.
you can see the lump by tail and the slight transparancy of the tail.

ta chris