A few pics... in feeling the koi, the bulge is very soft and "squishy", for want of a better word...

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I'm sorry to say your fish has a tumor. It's very common in female koi to get tumors of the reproductive organs. It could also be a very bad case of being egg impacted, but i's more than likely a tumor. In either case, there is no treatment.

Thanks for that information; since no treatment is possible, what's the prognosis (in terms of time left in my pond), considering that it was first noticed about 6 months ago?

It's progressing pretty fast. I have one who's tumor has not changed in 4 years. She may die fairly soon, or it may stabilize and allow her to live for a while. If it were my fish, I think I'd leave her alone as long as she keeps swimming and eating. Once she starts to isolate herself and stay near the bottom, it may be time to euthanize her. An overdose of clove oil (16 to 20 drops per gallon) is a humane way to put her down whent he time comes. Sorry I couldn't have better news for you.

Horrible news, but delivered in a very expert way, and the information really helps- thanks so much.

Anything to look for, in terms of why the tumour developed in the first place?

Glad I could help. There is no way to predict these tumors and no way to prevent them. No female fish is immune to them, be it a $5 koi from petsmart or a $100,000 show koi from Japan. Many people who don't show koi stick to buying all male fish though, to avoid just such problems. Male can get tumors too, but it's much rarer. Male koi are usually cheaper, easier to come by and more colorful at a younger age. They never develop the large torpedo shaped body that mature females get though, which is why males are not desirable as show fish.

I just wanted to say I'm sorry about your poor little fish. I agree. ccoombs certainly handled this in a professional and very helpful manner.

Ccoombs is an expert when it comes to Koi and she is probably correct in her diagnosis but one other option to consider would be Swim Bladder Disease since it is fairly common.

It would give the same bulge on just one side but usually the fish would swim with more difficulty, listing to one side or staying either on the surface or on the bottom. The difference is this condition can be treated. That is why I mention it.

A partial list of suggestions that could help:
"Feed your fish a couple of peas. That's right, peas. Just get some frozen peas, thaw them, and feed them to your fish. A professor of fish medicine at N.C. State College of Veterinary Medicine has done this in several cases with very good results. He thinks that the peas somehow encourage destruction of the impaction. No hard scientific data yet, but it's worth a try.
Fast your fish for a couple of days. Withhold all food for three or four days, and sometimes this alone will break up the impaction and return things to normal. Most fish can go a week to ten days without food and be just fine.
Periodic aspiration of the swim bladder works very well. Basically, you stick a needle in the swim bladder and suck out some of the air. Not something to be entered into lightly, but does work well. This is not a cure, but a successful treatment. The head veterinarian at the Baltimore Aquarium prefers this method.
Partial pneumocystectomy. This is another word for surgical removal of part of the swim bladder. I mention this less as a practical option but more just to let people know that there are vets out there doing X-rays, surgery, general anesthesia, even cancer chemotherapy."

I've never had a fish with a tumor but I have had one with a swim bladder problem. It resolved itself with the peas and gentle massage at the edge and lower back of the swelling. this might not help your fish at all if it is a tumor but it couldn't hurt. Some people prefer to cook the peas for a few seconds and then pop out the inside of each pea. Throw away the skin of the peas. It is tedious but my fish love them.

Here is a link that might be useful: never stick a swollen swim bladder with a pin! leave that to a vet.

Thanks, Cadence...

Sleepless, I think I'll try the peas; couldn't hurt.