Bloated koi. What can I do?

Growing large and voluminous koi is the objective of many koi keepers. Having purchased a beautiful specimen, the quest is to help the koi to grow to its full potential. These koi are provided with a high protein diet and an expansive pond to provide koi with healthy growing conditions. The pond may even benefit from an additional source of heating.

If all goes to plan, then the koi will increase in length and volume at similar rates to produce a fish that shows a body of good proportion.

There can be however, perhaps a single or even a number of koi in a collection that adopt an abnormal growth pattern, where the koi’s volume continues to increase without the associated increase in length.

Besides being unsightly, such koi will often swim with apparent discomfort, wriggling with difficulty and adopting a strange pose in the water. Such symptoms are quite common, especially in older fish, and can be caused in a number of different ways.

Causes of bloating in koi.

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.

Koi will respond to a number of environmental stimuli, such as sudden changes in temperature and water quality of simply the presence of suitable mature males. If these stimuli do not occur (which is not uncommon in the UK climate or in heated ponds where temperatures can remain unnaturally constant), then koi will not be given the means with which to spawn.

This phenomenon is easily diagnosed if only females appear bloated, and can be remedied in a number of ways.

Try a rapid 30% water change with cooler water and add spawning media

Contact a koi/carp farmer who can administer an ovulating hormone to stimulate the spawning response.

Kill blanketweed and string algae.