Black widow tetra need an answer?

i had black widow tetras for about 2 years last night i noticed one of the two remaining fish hanging about the top of my tank wich i found strange this morning when i went too feed my fish the black widow which had been at top of the tank had got black coloration back on its lower fin and tail fin then it died a couple of hours ago can anyone give me a reason for this water perameters are fine as are the other fish answer please...