My family has an approximately 8,000 gallon koi pond which has been established for 3 years. We inherited my grandfather's koi fish and now have 11 total, in addition to a handful of shubunkins/comets. Overall things have carried on smoothly! We absolutely adore our fish!
About 2 months ago we noticed our shubunkins/comets swimming/chasing particular fish around the pond, pretty ferociously. This resulted in one of the shubunkins/comets being partially out of the water and almost dead. Thankfully we were able to revive him/her and it is now in a hospital tank and has been doing very well! The side that was exposed to the air was "fluffy" and it has shed its dead scales! However, today I noticed that there were some clear bubbles collecting on its under belly on under its mouth/side of its face. We typically change out 1/2 of the 10 gallon tank once a week using the pond water....however, nothing in particular was changed from yesterday to today (didn't notice any unsual behavior or bubbles on the body yesterday). The fish is also visiting the surface frequently, which is also unusual (usually likes to stay toward the bottom of the tank, in the back corners). We have added another airation tube to the tank to see if that will help....but we are not sure! We are emotionally invested in the recovery of this fish and were hoping to reintroduce it to the pond next week, but now we are afraid that there is something else going on. PLEASE HELP!