Red Spot On Black Skirt Tetra

[Megsmelody]

I recently started up my fish tank. It is 5 gallons and houses 1 black skirt tetra and 3 cardinal tetras. Today I noticed that my Black Skirt Tetra has a small red spot on his "chin" so to speak. When I got him I got 3 Back Skirts, but 2 died the first day. I didn't think much of it, I assumed they were just sick to begin with or weren't strong enough to make the move from the pet store. But now that I see the red spot I'm concerned. Any ideas?

Thanks!