Red Blob On My Kerri Tetra?

[xamy_valox]

Hi there, just noticed a red blood looking blob on my Kerri tetra's back fin.
He seems to be darting around the tank quite a bit but then so do the others so I'm not sure it's a difference in behaviour, I'll try and get a pic though they move so fast it will be difficult.
Anyone know what it could be?
Amy xx