What makes you think he has fin rot I dont see anything but again I am new to Betta keeping. Please be sure and wait for others to have a look and see what they think before you treat him with anything. Post his pictures in the posts for Betta illness and see what others think. It came to me that Flame would be a good name when I saw the color on his fins. He is pretty ! There does seem to be a split in his tail fin maybe that is 3lizabeth is referring to. Best of wishes to you and you little guy !
I don't see fin rot, but a split is possibly from over flaring, but that happens fin rot in black at the edges, and I am not sure if that is his color, cuz fins would recede if they are truly rotted, or rotting, and it really don't look like that, I beleive that is his color..tho I cannot really make it out in the pix..sorry..in your Avi he looks fine..and gorgeous!!! and btw..Flame is a perfect name for such a showstopper with that bright red..:)
If you look at my Sammy's tail couldn't even tell what he was when we got him, cuz he was a terrible tail biter which i would of never known until I came to this forum, and learned all about it, but that is the way we got him, and now I just say oh well, I love him, and his chewed on tail..!!
Yeah lelei I am thinking its just his color too . PucaMama I would not worry about fin rot if I were you unless you see his fins beginning to get ragged looking and black along with getting shorter. He has a very nice home ! You can do a search here on Fin Rot so you will know better what to look for.
Check out the eBook Betta Fish Care Made Easy
how did he get that 
