Is My Fish Pregnant, Fat or Just Bloated?

Sometimes fish may appear swollen and larger than usual. It’s important to know why your fish has a swollen belly. So, how can you determine if your fish is pregnant, fat, or just bloated?

First things First:

You’ll need to identify the symptoms related to each condition. Fish may become fat or swollen due to pregnancy and develop a gravid spot. A bloated fish may have scales with a pinecone-like appearance. If none of these symptoms are present, your fish may just be getting fat. 

In this guide, we will learn to distinguish a pregnant fish from a sick or fat one. Let’s go!


One reason your fish may appear swollen is that it is carrying eggs. If your fish is a female, this is a very high possibility.

Not many fish species are live-bearing. Live-bearing fish can give birth to young called ‘fry’ rather than laying eggs in the tank. All of these varieties exhibit signs of pregnancy before the fry are born. According to Animal-World, the most common live-bearing fish species that can get pregnant are mollies, platies, guppies, and swordtails. Frequently, the mating rituals of these fish species are mistaken as aggression. Within about 20 to 40 days after the mating ritual, the female will develop a bulge in the abdomen.

Live-bearing fish can also breed easily. They can retain sperm and use it to become pregnant later when the conditions in their surroundings suit them. In other words, even if all your fish are female, pregnancy is a viable possibility.

Signs Your Fish is Pregnant

Check for a pregnancy bulge

The fish will grow a bulge at the back of its abdomen. The abdomen grows over the course of 20–40 days into a ”boxy” shape or a rounded shape.

Some fish species, such as balloon mollies, have a natural bulge below the gills.

Overweight fish may develop a bulge in the front chest. If you skip feedings for three days, a fat bulge may shrink, while a pregnancy bulge will not.

Look for a red or black spot

Pregnant female fish will develop a “gravid spot” on the abdomen near the rear vent. This spot is black or bright red and grows throughout the pregnancy.

Some fish always have this bright spot, but it will grow darker once the fish is pregnant. This spot in the fish will grow both larger and darker when fry is beginning to develop inside.

Swimming Difficulty

Some female fish have difficulty swimming when they are pregnant. This is because the fry developing inside is weighing her down. This may not happen to all-female fish, but it is often a telltale indicator of pregnancy.

Refusal to Eat

Another telltale indicator that a fish is pregnant is when she refuses food before giving birth. Unlike humans who enjoy food during pregnancy, fish refuse to eat during pregnancy.


It is normal for a pregnant fish to hide or explore plants and decorations in the aquarium. This is an indicator that she is ready to giving birth. Many fish look for a place in the aquarium to deliver their babies. This usually occurs one or two days before the labor process.

How to Tell Your Fish is Bloated  

Fish may appear bloated due to various reasons, including:


One reason your fish may be bloated is sickness. The main culprit for sick freshwater fish is Dropsy. Dropsy causes your fish to become larger than usual. So, what is Dropsy?

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Dropsy refers to a disease in which fish have a swollen belly. It causes fluid retention inside the fish’s body. Fish with this condition may seem bloated.  

Dropsy in fish is a cluster of symptoms caused by an infection from bacteria commonly present in freshwater aquariums. Consequently, your fish may be exposed to the dropsy-causing bacteria. Healthy fish rarely fall prey to the dropsy disease. Fish are only susceptible when they have a weak immune system. Stress can weaken a fish’s immune system and make them more vulnerable to disease. If all the aquarium fish are under stress, it’s quite common for them to become infected.

Signs of Dropsy in Aquarium Fish

As the infection progresses, the belly may fill with fluids, and skin lesions may appear. Once the infection becomes severe, the internal organs get damaged, and the fish dies. Even with proper treatment, the mortality rate of dropsy-disease is high. Successful treatment is rare unless you can diagnose a fish in the early stages of the infection.

Symptoms of the underlying bacterial infection vary widely. Some fish have the classic swollen belly, and others display skin lesions. Some fish may show no symptoms. This variability makes diagnosis difficult, but you can observe physical and behavioral symptoms in most cases.  


  • Swollen belly
  • Scales with a pinecone-like appearance
  • Bulging eyes
  • Pale gills
  • Red and swollen anus
  • Pale and stringy feces
  • Ulcers on the body
  • Curved spine
  • Fins clamped together
  • Redness of the fins or sin
  • Lethargy
  • Refusal to eat
  • Swimming near the surface


If your fish has these symptoms, you may want to quarantine it from others. Dropsy is an infectious disease and can spread. Dropsy is fatal, but you may want to talk to your vet about possible antibiotic treatments that can save your fish.

Some experts recommend euthanizing the affected fish to prevent the spread of the infection. If you detect the infection early, you can isolate the fish for proper treatment and recovery. Treatment can correct the underlying problems and provide supportive care to the sick fish.

Here’s what you can do:

  • Move the sick fish to a “quarantine tank.” It is important to move the infected fish to another tank to curb disease spread. You can perform a water change on the original aquarium and monitor the remaining fish closely for any symptoms.
  • Add one teaspoon of salt per gallon of water to the quarantine tank.
  • Provide the sick fish with a variety of fresh, high-quality food. Often this is enough to treat the infection. Keep the fish under observation for a few weeks after the symptoms disappear.
  • Treat the infected fish with antibiotics, either in the water or in the food.
  • Test the water in the quarantine tank daily to ensure it is appropriate for the infected fish.
  • Use antibiotics if fish doesn’t respond to regular treatment.

Note: Don’t add too much salt. Low salt levels in the water improve the osmotic balance and help the fish get rid of excess fluids accumulating in the body, causing Dropsy. Too much salt can be unhealthy for freshwater fish. Keep the quarantine tank clean, and perform weekly water changes.

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What other Diseases cause Bloating?

Kidney Disease

Other possible ailments that can cause bloating are kidney diseases. The main function of kidneys is to remove access water from a fish’s body. If kidneys aren’t functioning correctly, excess fluids can build up internally and cause bloating. Signs of excessive water retention are rounding and swelling of the belly.

There are many environmental factors that can affect the function of the kidneys. It may malfunction because of an infectious or non-infectious cause. Kidney disease can also be related to stress.

In addition to removing toxins from the body, fish kidneys are responsible for making white and red blood cells. Stress can cause extreme changes in the production of these blood cells, affecting immune system function. Poor water quality, noise pollution, improper diet, and overcrowding can cause higher stress levels in fish. All of these factors increase disease susceptibility and spread and can affect other organs.

Bloating is a symptom of underlying diseases, so you shouldn’t confuse it with pregnancy. Consult a veterinarian if you believe that your fish is sick.


A tumor is another cause of your fish looking larger or bloated. Tumors are usually uneven and are benign. Fishes can develop tumors, much like humans and other animals.

Symptoms and Types

Most tumors are seen as bumps under the fish’s skin. The tumor location can be different for each fish and depend greatly on the tumor type. You may confuse a tumor for pregnancy, so it’s important to consult a local vet and get a diagnosis. Unfortunately, internal tumors display symptoms once it is too late to save the fish. The fish’s ability to eat and swim will also be impaired, causing a rapid decline in the fish’s health.

Some fish get tumors in the reproductive organs. You’ll notice a swollen abdomen, and the disease can become terminal.


Most fish have a genetic predisposition for tumors or cancers, and others can get tumors from a viral infection.


Most tumors found in fish have no cure or treatment. Internal tumors are also not identified until the advanced stages of the disease. And when it is diagnosed early, the position of the tumor often makes it inoperable. This is the main reason most fish with tumors and cancers are euthanized.

How to tell if your fish is fat

Sometimes a fish may become swollen or fat as a result of overfeeding. Often, overfeeding can cause constipation and other digestive issues. You can feed your fish less or healthier types of food, which may vary depending on the fish species. Overfeeding also has more serious consequences than digestive ones. More food means more waste – overfeeding increases ammonia and decreases oxygen levels in the water. If you see your fish gasping at the water surface, this means that the water quality is poor. So, you may want to reduce overfeeding.

Final Thoughts

If the increase in a fish’s size does not appear to be the result of any of these factors, it is possible that your fish is healthy and simply growing. If you have concerns about the health of your fish, it’ll be wise to contact a veterinarian.

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Nadine Oraby

My name is Nadine; I am a passionate writer and a pet lover. People usually call me by the nickname “Joy” because they think that I am a positive and joyful person who is a child at heart. My love for animals triggered me to create this blog. Articles are written by vets, pet experts, and me. Thanks for visiting. Your friend, Nadine!

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