White Lump on tail?

Question
I have a one and a half year old beta fish.
He is otherwise healthy, eats well, water is cleaned on a regular basis, swims around quite well, fins in tact.

In the past two weeks he has developed a white-ish, bumpy, cottony type growth right above his tail fin.
It seems to be growing quickly.

Any thoughts of ideas on what this could be?
Any treatment options?
Is it fatal?  Did I cause it?

Thank you,
Sage and Sheldon (the fish).

Answer
Sounds like a fungus.

There are many medications on the market, just go to your local pet store and pick up a medication that treats body fungus.

Not fatal if treated, but ya, you did cause it. Fungus is caused by extremely poor water quality, so I'm guessing your water wasn't as clean as you thought it was.