Koi Ponds Goldfish Ponds & Bloated Fish

Page Summary: Have you ever had that "bloated feeling". This is an apt description of a fish swimming around that appears too fat for comfort. The koi or goldfish seems to have been "blown up" like a balloon. We associate bloating (not sure if thats a word but you get the point) with over-eating. This is not the case with koi or goldfish. In fact bloating is a symptom of a disease or in springtime an indication that a female koi is ready to spawn. This article deals with bloated koi in terms of a koi or goldfish being sick and not pregnant. Bloated koi will normally have difficulty in swimming around the fish pond.

My Koi or Goldfish Seem Bloated

Anybody who has overeaten knows how uncomfortable it is to feel puffy and bloated. But koi dont only become bloated if they overeat. In fact pond fish generally dont overeat. This is why it is so important not to overfeed them – because the excess food will usually remain in the water and pollute it.

So what does make koi appear to be bloated? There are several reasons including:

  • spawning,
  • infection or malfunction of the body organs,
  • tumours,
  • obesity.


Like human females, female koi fill out in size when they are pregnant. This will give them the appearance of being bloated. If you are going to allow a spontaneous spawning, then there is nothing you need to do. If the water temperature is right, and males are present in the water, the female koi will lay her eggs. It may, however, be a good idea to add spawning media to the pond, for her to deposit her ova into.

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If you are going to prevent a spontaneous spawning event (which most serious koi keepers prefer), then this is the time to ensure that males are not in the pond with the females. If the males are separate (which they should be), then this will be the time to induce the female, and artificially release the fish eggs (or to call someone in to do it for you). Then you will also need to collect sperm from the male koi so that the ova can be fertilised.

Infection or malfunction of the organs

Sometimes koi do get sick, especially when they are older. This may result in them looking uncomfortable and bloated. They may also seem to have difficulty swimming.

A build-up of excessive body fluid around the tummy can make koi look bloated. This may be the result of a bacterial infection within the body, or it could be because the kidneys are not functioning properly. In severe instances, the eyes will also tend to protrude. Generally the diagnosis will then be "dropsy".

It is important to establish whether the fish is sick or pregnant. If it is older and only one is affected, there is a good chance that it is in fact excessive tissue fluid that is causing the bloating effect. In this case you need to remove the affected koi to another pond and treat it. If the bloating does not subside, it should be euthanized humanely.


While a fish with dropsy will appear considerably more bloated than one with a tumour, bloating may be a symptom of a tumour. Generally any tumour will first appear as a lump which will then continue to swell.

There is little that can be done to cure a koi with a tumour. However it will be less of a threat to the other fish in the pond. Euthanasia is the most usual solution.


Having said that koi dont generally over-eat, it can happen. It is therefore your responsibility, as a keeper of koi, to ensure that your fish are not over-fed. It isnt just the volumes of food you need to consider, but also the protein content. If your koi seem to be looking fat and bloated, and you are able to eliminate the other possibilities, then put your fish on diet. This should not entail cutting the amount you feed. What you need to do is switch to the low-energy diet fed to koi just before the onset of winter when they dont get fed at all.

Youll be amazed at how simple it can be to breed fish with the help of spawning brushes. Spawning BrushYoull need one mature female and at least two males.

Put the brushes one above the other at the edge of the pond. Spawning usually occurs in the early morning after a night of flurried activity and can take several hours. Most of the eggs will stick to the brushes which can then be taken out of the pond and put in filtered water of the same temperature. If the eggs are not separated from the adult fish they may be eaten.

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