What Is This?

[Kairi]

who knows. he was acting a little weird again yesterday, but is fine today. (no swimming upside down) i think they just like swimming vertically sometimes.

 

[Torrean]

tsi I believe a kidney stone would be hard. Not likely to be a poppable sack like this.

Not seriously suggesting this cause that fish would be in serious trouble but swim bladder?

What was the sac filled with?