What is this?

[Kinsie]

My 1 year old male guppy has a weird lump in his tail. It is pretty big. He doesn't act like it's affecting him at all. He has had the same tank set up for 2 years it's a 20 gallon. They eat a variety of froazen brine shrimp, brine shrimp dry, algae flakes, and fish flakes.


Any ideas to what can cause this can cause this and what it is?

I will post pictures first thing tommorow.

 

[Kinsie]

 

[Demeter32]

Looks like it's growing in the base of the tail and not the actual fin part. This could mean it's a tumor of sorts. Not too sure on what you can do at this point. He looks very happy and healthy otherwise. You will probably just have to wait it out and hope it disappears over time.