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7 Reasons Your Koi Fish Has An Open Mouth!

Although we would never recommend keeping koi to beginners, the popularity of koi within the fish keeping community is increasing rapidly right now.

Koi can be a great fish to keep in the right setup but they can be problematic for experienced fish keepers, never mind beginners so we often see people reaching out with various questions about problems they are having with their koi fish.

One of these problems that seems to be very common right now is issues with a koi fish having an open mouth constantly.

Not only can this be due to a potentially serious issue but it can also cause additional problems with the fish further down the line too.

If you have noticed that your koi fish does have an open mouth every time that you see it then you really do need to fix the issue as soon as possible.

With any luck, this article will be able to help you better understand the most common problems that can cause your koi to have its mouth lock open.

Some of these issues are very easy to fix where as others can be more difficult and may require the assistance of a vet or an experienced koi keeper to help correct the issue in the fish properly.

Why Your Koi Fish Has Its Mouth Open!

Here is our list of the more common reasons that your koi fish may have its mouth open but we will explain each of them in more detail later:-

  • Injury!
  • The Jaw Needs Repositioning!
  • The Koi Is Feeding!
  • Infection!
  • Abnormalities!
  • Too Much Fat In The Diet!
  • Unsuitable Food Size!

Although very rare, some koi can actually have more than one of these problems wrong with the fish at the same time so keep that in mind when you try to work out why your koi can’t close its mouth.


One of the first things that you are going to need to check for if your koi fish has an open mouth is any injuries that may have caused this problem.

If your koi fish has been in a fight or you have seen another fish attacking it then there is a chance that the koi could have received an injury to its jaw.

If this is the case then you are going to need to carefully check both the top and bottom jaw of your koi fish for any cuts, grazes, swellings or anything else that looks abnormal.

If you do find something then you will need to treat the injury as soon as possible otherwise it could become infected which would cause even more problems for the fish.

The Jaw Needs Repositioning!

Another common reason that koi fish can have an open mouth is due to the fact that their jaw has become dislodged and needs to be put back into place.

This is actually quite a common problem in cheaper koi with poor breeding lines and one that is relatively easy to fix too.

If you think that this may be the problem with your koi then all you need to do is place your thumb and forefinger on either side of the koi’s head just behind the gills.

From here, you simply need to apply a little pressure until you hear or feel a ‘pop’ as the jaw snaps back into place.

If you are struggling to do this or you are worried about damaging the koi then we would recommend taking it to a local fish store or vet for assistance.

The Koi Is Feeding!

We know that this one sounds basic but it is surprisingly common for people to think that their koi is having problems with an open mouth when in actual fact the koi is simply feeding.

If you are accidentally underfeeding your koi then it can be common for them to forage more frequently and they may have their mouth open for this too.

This can give the impression that the koi is keeping its mouth open more frequently than it should but the fish is just trying to find food.

You can usually test if this is correct by giving that specific koi more food to see if it closes its mouth after having its fill.


If your koi fish has an open mouth then there is also a chance that the issue could be caused by some sort of infection.

The most common type of infection that can cause this problem is actually mouth rot which is a bacterial infection that causes the gums and tissues around the mouth to die.

This problem is relatively easy to spot as the tissue around the mouth will look red and inflamed and there may also be some pus or white patches visible too.

If you think that your koi has mouth rot then you are going to need to treat it as soon as possible otherwise it could spread to other parts of the koi’s body and potentially kill the fish.


In some rarer cases, a koi fish may have an open mouth due to some sort of abnormality.

One of the more common abnormalities is actually a cleft palate which is where the roof of the mouth hasn’t formed correctly and leaves a split or hole.

This problem is usually genetic and quite rare but the cheaper koi farms tend not to maintain strong breeding lines so the issue is becoming increasingly common with cheaper koi.

Too Much Fat In The Diet!

Too much fat in the diet of a koi fish while it is growing up can result in a number of problems with bones and cartilage in later life.

One side effect of this is the fish having issues with being able to control the opening and closing of its jaw.

Unfortunately, there is usually nothing you can do to fix this problem and in the majority of cases, it is an issue from before the koi fish was even in your pond.

As you may guess, this is usually from the cheaper koi farms who use cheaper foods with unsuitable macronutrient breakdowns to feed their young koi.

Unsuitable Food Size!

Although koi are large fish, the pellets that they eat can’t be too large or it can cause issues with them over-stretching their jaw to feed.

This can also be a problem if you accidentally add too many pellets to your koi pond and your koi open their jaw too much to get as many pellets as possible all at the same time.

Switching over to smaller koi food pellets and offering less pellets at the same time should be enough to prevent this risk to your koi fish though.