bn Pleco has a new spot.

Hi, I'm a fairly new member here, but I've been lurking a while. I appreciate the wealth of info I've found on here.

I've had a handful of freshwater aquariums in my lifetime and after a 12 year hiatus I am back into the hobby. This time I've gone the planted route, and I really like the live plants.

I currently have a $10 10g Craigslist special with 9 LED low watt bulbs, a 50w heater, sand substrate, a HOB and a sponge filter.

There are three different plants: some elodea, maybe some kind of Anubis that has been producing a huge new leaf every week, and some mystery grass that probably needs much more light and fertilizer.

Anyway, on to my question: I have a LFS described blue eyed lemon bn pleco. She recently developed a small black spot on her tail fin. It's been there for nearly two weeks and has grown slightly. Does anyone know what it might be? She has a willow branch to munch on, algae wafers to eat, and blanched veggies every so often. She seems to be a very healthy and active pleco.