Why is My Fish Bloated?

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Bloating in our pet fish is one of the most obvious signs of problems, but not every fishkeeper is prepared to handle the problem. There are a few key causes to look out for, and if you know the symptoms well you’ll know which course of action to take.

So, let’s take a look at why your fish is bloated and what you can do about it.

bloated fish


Is My Fish Bloated?

Bloating is a serious issue, but not every fish that’s getting bigger has a problem that requires drastic intervention.

When you examine your fish you need to look for more than just a bit of a belly bulge. Take note of any of the following symptoms as well:

  • Ulcers or sores
  • Fins rotting
  • Eyes bulging
  • Lethargy
  • Scales being pushed outwards (“Pinecone Body”)

These are all signs of something more than overfeeding or constipation going on and will require a more detailed response to treat.

What Causes Fish to Bloat?

A bloated fish can be caused by quite a few problems. You may not always be able to distinguish between them, but below I’ll help you figure out which is the right course of action for your fish. Diagnosis is the first step, so you’ll need to pay close attention.


Overfeeding can cause your fish to appear slightly bloated, and it’s not great for their long-term health. Fish often need much less food than we give them, and this can also contribute to bad water chemistry like high nitrates.

It’s easy enough to tell if overfeeding is the cause: the fish will have no other symptoms and should recover after a 24-72 hour fast with no problems.


When overfeeding or an improper diet has become long-term, your fish may become obese. While not the worst cause of bloated fish, by any means, this will shorten their lifespan and lower their activity levels.

Cutting back portions will often take care of the problem. People often forget just how small their fish’s stomach is and overfeeding is a regular practice in a lot of tanks.

If you’re mixing up food it’s worth looking at what you’re mixing in with the staples. Bloodworms and beef hearts, for instance, can cause obesity if fed too often.

If you feed live foods, it’s worth looking at what you’re putting into the tank as well. Goldfish are commonly used as feeders, for instance, but are very fatty fish and shouldn’t be fed often. You may need to find a leaner alternative such as Ruby Red Minnows, or simply quit feeding live foods altogether.


Constipation happens to fish on occasion, usually a result of their diet. While it’s a common problem, and one that may resolve spontaneously, it can kill your fish if it’s not treated in time.

One of the biggest mistakes in the diet of a constipated fish is having carnivores eat algae wafers or other plant-based food.

In general, fish with constipation will have quite a bit of bloating, but no other symptoms except lethargy. You may also notice they haven’t released any waste in a few days, which is the best sign that you’re dealing with constipation.

Constipation is easy to treat, and often people will treat for it first before moving on to other methods if the problem persists. The big thing is to make sure that it doesn’t happen again.

Bacterial Infection

Some bacterial infections can cause a fish to bloat, and they may be treatable depending on how advanced they are. Differential diagnosis of bacteria isn’t going to happen at home, even just identifying the infection properly may not be possible.

Infections will cause secondary symptoms. These include fin rot, red spots, and even ulcers or sores on the fish’s skin. In general, bacterial infections kill a lot of fish and it’s important to make sure you’re stocked and ready to take care of them.

Fungal infections can be confused with bacterial, but are far less likely to induce bloating.

Parasite Infections

Some worm infections can present as a bloated belly, as can a few other parasites that fish are prone to. Worms can be differentiated easily since the infected fish will be passing them.

This doesn’t happen with all parasites, but a fish that’s resistant to antibiotic treatment may respond to an antiparasitic treatment.

There are specific products out there for handling parasites, separate from the treatments for fungal and bacterial infections.


Dropsy is more of a symptom than anything, usually presenting as a very advanced bloating. This is actually due to fluid retention in the fish, caused by a number of really nasty diseases and disorders.

The most common way to differentiate is to look at the fish from above. In cases of dropsy, you’ll usually be able to see the scales have turned outward due to bloating, causing it to look a bit like a pinecone. You may also see swelling around the anus and pale gills.

Previously this condition was considered a death sentence for the fish, but that’s not quite the case today. Instead, an Epsom salt treatment is used before euthanasia. It seems to provide some help and support and may let the fish survive until the condition passes.

In the case of smaller fish, by the time this symptom presents the condition is advanced and it may be best to simply euthanize the affected fish.

How Do I Treat Bloated Fish?

Once you have some idea of your fish’s overall condition, it’s time to begin the appropriate treatment. Without a diagnosis from a vet, who will prescribe the appropriate medications, you’ll have to rely on standard protocols to try and save your fish.

The important thing is to act early.

Dietary Changes and Solutions

While most fish are fairly simple to feed, you can run into problems if you’re not careful.

Overfeeding can be solved in a simple way: just lower the amount of food that you’re feeding. Less flake, fewer pellets, or thawing out frozen foods to feed them over 2-3 days instead of all at once work well enough.

For fish that are constipated the usual answer is to feed them peas. Peas have high fiber content and tend to loosen up the bowels of the fish, getting their digestive tract flowing normally again.

Some fish need a particular diet, or may just have problems if fed too much of any given food. Try to balance their diet based on known information about their wild diet for the best results.

Medication for Infections

A quarantine tank is a must for serious fish keepers. I usually keep a 20-gallon tank with no substrate and a few fake plants cycled and ready to go just in case. You should also use one before introducing new animals to their final aquarium since it will let you know if they’re sick or not.

Sometimes it makes sense to treat the display tank, especially if more than one fish is affected.

Fish suffer from three main sources of infection:

  • Parasitic
  • Bacterial
  • Fungal

Viral infections can also occur, but there’s no real treatment for them.

Diagnosing a fish is dicey at best, most of us aren’t specialized veterinarians after all. In my experience, bacterial infections seem to be the most common. Or at least fish seem to respond to antibacterial medicines the best.

The usual fish medications work very well, provided your diagnosis was correct. Because it can be a bit of a shot in the dark, it’s often best to use combined medications. I advise using these only in a quarantine tank, but you can use your main display if you don’t have access to one.

My preferred brand is SeaChem’s KanaPlex but I’ve also used API’s Melafix with good results in the past.

Follow the instructions from the manufacturer to the letter and hope for the best. It’s also best to temporarily remove any activated carbon in your filters. They may pick up the medicines and prevent them from being in the water column.

I’ve never had a confirmed interior parasite infection in any of my animals, but it does happen. Worms are the usual cause of bloating if it’s a parasite, so a deworming agent is your best bet. The one linked is safe and sometimes used as a preventative, be careful with others as they may alter pH or otherwise be unsafe for your fish.

Epsom Salt Treatments

Dropsy is a build-up of fluids inside the fish’s body. In most cases, it’s a death sentence for the fish. I’ll admit that I’ve never had a fish recover when it’s reached the point that it has a pinecone body, but anything is worth a shot.

Larger fish are more likely to recover than smaller ones. I advise euthanizing any fish under 4-5” that’s displaying pinecone body as a symptom since they’re in the advanced stages of the disease. With larger fish, it’s easier to catch early.

The idea behind Epsom salt treatment is to take advantage of biological osmosis to reduce the fluids in the fish, and hopefully allow it to live long enough to recover. In layman’s terms, a fish in salty water loses water as it crosses through the cell membranes of its skin. A bath is a better idea than using your main tank, but either will work.

If adding to the main tank, ensure that you don’t have any fish or invertebrates sensitive to salt changes. Most will be fine, but it can cause problems with others.

The better way to use this magnesium salt is to create a temporary bath that you’ll place the fish in for 5-30 minutes.

Dosing is relatively straightforward:

  • In a quarantine or display tank use ⅛ teaspoon per 5 gallons of water.
  • In a bucket for short baths use 1 tablespoon per 1 gallon of water.

Epsom salt has the side benefit of being great at handling constipation as well, which will cover another cause of fish bloating.

Using Epsom salt baths with the fish in a quarantine tank treated with medications will cover most of your bases and give your little buddy a fighting chance.


Even the best treatments sometimes fail, which is why we sometimes have to make hard choices. There’s no point in prolonging a fish’s suffering, and when it’s clear the fish will die we can help them along to make it a little bit more comfortable.

There are a few methods used, but I recommend using clove oil unless you can’t find any at all.

Clove oil preparations are simple and work quickly. You’ll want to add roughly .5mL per gallon of water in a bucket, then add the fish. Clove oil is used for anesthesia when surgery is required on fish but this is a fatal dose and will result in the fish “going to sleep” in a peaceful manner.

Freezing is another method used. Take a bag or bowl of water and place it in the freezer until it’s a bit slushy, then place the fish inside and put the water back in the freezer. This is humane but can be impractical for larger fish.

The final method is a bit grisly, but it is quick and relatively painless for the fish. Decapitation and pulverizing the brain is quick and easy but it’s a hard thing to do to a beloved fish.

You can also go down the ikejime route, which is a Japanese method of killing fish quickly for consumption. This requires a deft hand and a bit of extra knowledge about the fish’s anatomy, but the general principle is to drive a spike through their hindbrain.

When done properly spiking a fish is very humane. The fish will simply relax after having its nervous system shut off, though there may be a small amount of movement. To avoid guesswork, find a fish in the same genus on this website to make sure you get it right on the first try.

Putting down a fish is never a great way to end the day, but ignoring their suffering is a much crueler way for them to die. That’s why I recommend all aquarists keep clove oil on hand in case an animal needs to be euthanized. 

One Step at a Time

A bloated fish is a major cause for concern in your tanks, and immediate action needs to be taken to correct the problem. Medications, dietary fixes, and Epsom salt baths can give your fish a fighting chance, but in advanced cases euthanasia may be your only option. With so many causes for bloating and dropsy, it’s almost impossible for the layman to make an accurate diagnosis at home.

That said, the first step is fighting back with what you know. Take action now rather than later, it’s the best chance you have at helping your pet survive.