Hi, sorry about the picture quality, I tried my best, and I only had my Ipod to take the picture, anyways, notice the kind of dark area I circled, is there some kind of disease? I noticed it being there just today... and it's 'growing' right after the tail fin. (I bought this female guppy from a pet store not long ago, I've had it for around 2weeks~) Is this normal, if not, what kind of disease is this, and what should I do? Thanks in advance!

