What On Earth? Bubble In Betta's Fin?

[dylema]

Very weird. We just looked at our fighter and it seems he has a bubble inside his dorsal fin? Quite large, maybe 3-4mm across? Looks perfectly healthy otherwise!
Do you think it's air or fluid in the bubble? Whatever it is it's less dense than the water because it's floating. Doesn't seem to impede his swimming too badly.
I thought I saw this once before a few months ago but it went away within a day.
Got a few pics to show you what I mean...

Thanks in advance

 

[dylema]

haha very funny.

It's shrinking, already half the size less than half an hour later

 

[dylema]

This can happen by changing too much water at one time or by adding cold water to the fish tank

Haven't done either of these things. No water change for a few weeks (big tank) and very careful about temp of his water too
That doesnt' sound very nice though!

We've added a little melafix just in case (low dose)

 

[WILDER]

Are you running an airstone.

 

[dylema]

he's got a filter with a spray bar (pointed at the wall) so should be getting plenty of air. The bubble has gone completely now but I'd still like to know what caused it!

 

[blitztidus]

That could just be a wandering bubble that he got from getting too close to that, that happens to my angels sometimes too. They like to get right in it and get bubble all over them.

 

[Tokis-Phoenix]

Could have been a cyst, usually filled with fluid or pus and result from bacterial infections or general infected infections from injurys like a puncture wound. Usually they don't go away on their own without some sort of aid like a med, but its not imposible.