I have a large Oranda (at least I think he is, large orange head). He has
what looks like a blister on the inside of one of his gills. It's swollen
on the inside, and a bubble has formed that sticks out of the gill cavity.
I see no signs of blood in it, and it looks totally clear inside.

Any idea what this is? Should I leave it, or try to lance it?

No other problems with the fish. I have 4 in a 40 gallon tank. All seem
healthy, happy, active, and eat well.

Thanks for any help,

Devin