Glofish Has Abcess (?) on Anal Fin \ Tail

[TXStankyHank]

My son has a glofish that has what appears to be an abscess on it's anal fin. The fish is eating as normal, swimming as normal. I tested the water and everything appears to be in good condition.

We're curious as to what it could be, I have had fancy goldfish (Orandas and Ranchus) for years and have never seen anything along these lines. I attempted to find similar pictures in this forum as well as through google images, but I did not have any luck.

Thanks in Advance!