Newbie Question, Goldfish With Lump

Hello group!

I have a question and any help saving my little friend would be greatly
appreciated.....

My approx. 3 year old goldfish (about 6 inches in length) has developed a
small lump right behind his left fin. he has had it for about a week now.
It looks somewhat similar to a pimple. It was red the first few days, but
now it seems to have gone back to his normal color. It grew for the first
few days, now its been the same size for a few days. He doesn't seem to
effected buy it (his behavior is the same), but the lump looks to be
growing. The 20 gallon tank he is in alone has been running for about 6
months without any problems, and the PH is neutral. The water is clear
without any clouding at all.

Does anyone know where I can get some info on how to fix this condition? I'm
pretty clueless about goldfish in general other than basic tank maintenance.
Any info at all would be a huge help to me!

Thanks for reading!
-Patrick