Your fish may have one of the follwoing conditions:
1. Collection of excessive tissue fluid.
The 'belly' of a koi can be caused to bulge on account of a collection of tissue fluid around the abdomen area. This can be caused by an internal bacterial infection or the malfunctioning of the kidneys. Behaving like a fluid-filled balloon, the volume and pressure within the koi can cause blood capillaries to become visible in the skin and the eyes to protrude.
This condition can easily be misdiagnosed as koi being spawn-bound, but if scales protrude from the sleek lines of the koi's body then this is a positive identification of a koi suffering from dropsy. This is quite terminal and can pose risks for other koi in the same pond. The affected fish should be isolated and monitored if possible and put to sleep with an overdose of anaesthetic if the symptoms deteriorate.
2) Tumour
A fish suffering from a tumour is not likely to become as apparently 'full' as a dropsied koi and is only likely to affect a localised area of a fish. A tumour is likely to manifest itself as a lump, where the swelling may even appear to one side if the koi is viewed directly from above.
As with dropsy, there is little remedial action that can be carried out, but it is less likely to affect the other fish should it develop further.
3) Obesity
A 'bloated' koi may just be that! In the race to grow koi as quickly as possible, it has been known for koi to be fed excessive quantities of food that is too energy-rich for koi, being deposited around the gut and internal organs as fat.
If this is the cause then the majority of koi in a pond are likely to exhibit obese symptoms. The remedy is to put your koi on a diet. Change their diet to a low-energy autumn/winter food causing the koi to utilise energy that is stored in their fat.
4) Spawn
Mature female koi will naturally 'fill-out' from spring through to summer, when, if the stimuli for spawning occur, the koi will spawn naturally in the pond. However, it is quite common for older female koi to reach July or August without spawning, with the likelihood of spawning reducing every day. sourced from pond-doctor.co.za
Joe, I believe your koi is suffering from dropsy being an internal bacterial infection, causing tissue fluid around the abdomen. There is a risk that it might affect the other fish in your collection, and dropsy can not really be treated although high dosages of Acraflavine together with salt or antibiotic injections CAN in some cases stop the infection. Uours has developed quite far. In view of the very slight possibility to rectify the problem and the risk of infecting other fish, I would consider to cull it.