Betta Fin Problem

[Marnee]

There seems to be a bubble like growth inside the top fin of my Betta.  Started out smaller but has grown over the past week.  It almost seems like his top fin has become stiff.  I checked it with a magnifying glass to see if there was any fuzzyness to it thinking it might be an injury that has gotten infected and developed fungus, but there is no sign of any fuzzy area at all.  
He is in a 50 gallon planted tank with panda cory's, glowlight tetra's and rummy nose tetra's.  None of the other fish seem to be interested in him, or vice versa.  
 

 

 

 

[cpwebsite]

Nice tank.
 
My first guess would be that it could be fin rot. However, what I think it is is that something caused his fin to become injured, and that injury developed into a blister (maybe something like a blood blister?) However it could have also been from a bubbler in your tank that maybe got caught some how on his fin (if you have a bubbler), for example maybe he tore it and the bubble got inside. I'm just giving out some ideas.
 
For now, until you figure out what it is, make sure he has pristine water conditions with ammonia & nitrite @ 0 and nitrate <20. Try a 50% daily water changes. From my experience, even if you don't know what the problem is, water changes are definitely the best place to start.