Why is my Zebra Danio so Fat and Bloated? Is it Pregnant?

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It is pretty common to see changes in appearance among aquarium fish. For example, quite a few times, I noticed that my zebra danio becomes fat and bloated. At first, I wasn’t sure what caused that change. However, over the years, I gained some knowledge on the topic.

Zebra danios typically become fat and bloated due to pregnancy, in which they will also become very active, develop and bulge in the midsection of their abdomen, and appear relatively wider. However, danios may also appear fat due to overfeeding, constipation, Dropsy, and tumors.

As we proceed, I will show you how to deal with a bloated zebra danio, including testing the water with the API Freshwater Master Test Kit (link to Amazon). I will also teach you how to recognize a pregnant zebra danio, followed by a detailed video.

Why is my Zebra Danio so Fat?

Do fish get fat? Well, not in the wild, or at the very least, it is rare. This is because food is not as accessible in their natural habitat. Nevertheless, the same cannot be said for fish in aquariums. Aquarists overfeed them all the time.

And unfortunately, fish do not refuse food. They will keep eating if you keep feeding them. But can you blame weight gain in every zebra danio on overfeeding? Not necessarily. Like all fish, zebra danios can become fat and bloated for any number of reasons, including:

1. The Zebra Danio is Pregnant

Zebra danios are not live-bearers. The females lay eggs that a male zebra danio fertilizes. The presence of eggs can cause a female danio to gain weight. To determine whether or not your fish’s bloating is a consequence of breeding, you have to account for the following factors:

  • Gender– Is the fat fish female? Female zebra danios are larger and fatter than their male counterparts.[1] They are also less colorful. If you cannot differentiate between your fish, place them on a rich diet consisting of premium live food for a few days. The differences in size between the male and female fish will become more pronounced.

Here is a Youtube video that illustrates the differences between male and female zebra danios:

  • Temperature – Where the temperature is concerned, zebra danios tend to spawn in the low-mid 70s (Fahrenheit). Get a thermostat and check the temperature. If the water exceeds that range, the chances that the female danio is bloated because it is carrying eggs are low.
  • Eggs – Look for eggs at the bottom of the aquarium. If you find eggs in the tank, you can conclude that your bloated zebra danio was pregnant.

2. Your Zebra Danio Was Overfed

This issue has to be emphasized because it is dangerous. Overfeeding isn’t a mere inconvenience. It won’t stop at simply causing weight gain. It is a health hazard that can attract various ailments, including constipation and swim bladder disease.

Fortunately, in many cases, overfeeding has to persist to produce lasting consequences. You are discouraged from assuming that all bloating in zebra danios comes from overfeeding. 

I suggest that you keep an eye out for specific signs, including:

  • Duration – Bloating resulting from overfeeding doesn’t last long. The zebra danio in question should regain its original size and shape within a few hours. If the bloating persists, look for additional causes.
  • Behavior – Are your fish always hungry? This can point to overfeeding, especially if you have taken a moment to observe the fish during mealtimes to ensure that they eat all the food you provide.[2]

Fish can act like they are always hungry even when they are not because they have learned to associate your presence with food. This can happen in situations where an aquarist adds food to the tank whenever they appear.[3]

  • Food – Check your tank carefully. If you can see food on the substrate, you are giving your fish more food than they can finish. Food items like pellets will float on the surface for some hours before sinking.
  • Water Quality – Overfeeding can cause the quality of your tank water to deteriorate. When leftovers in the aquarium rot, they cause ammonia and nitrite spikes. This can also happen because the zebra danios produce a lot more waste. That, in turn, rots and produces ammonia and nitrite spikes.

3. Your Zebra Danio Has Dropsy

Several diseases and infections can cause bloating in zebra danios. But the most prominent concern is Dropsy. The ailment causes fluids to accumulate in cavities like the abdomen, causing the fish in question to swell.

Dropsy is easy enough to identify. The scales will stand out, taking on the appearance of a pinecone.[4] Additional symptoms include a curved spine, lesions on the skin, loss of appetite, pale gills, and lethargy, to mention but a few.

The illness has been traced back to a bacteria called Aeromonas. Though, poor water conditions can exacerbate the illness. Dropsy is dangerous because it harms the internal organs.

4. The Danio Developed a Tumor

This is one of the worst-case scenarios. Tumors can cause a fish to bloat in ways that give its body an uneven shape.[5] Some tumors are benign. But others are cancerous. Short of removing them via a surgical procedure, tumors in fish are rarely treatable.

What to do With a Fat Zebra Danio?

Your response to a fat zebra danio will depend on the factors that caused the bloating. You have to match the treatment to the cause:

1. Adjusting the Aquarium Conditions

If your zebra danio seems bloated, I highly suggest that you improve the water conditions. It will promote the fish’s general state regardless of the cause that made it fat. 

That means maintaining a temperature of 60 to 75 degrees F, a pH of 6.0-8.0, and hardness of 2 to 20dGH.[6] Also, the tank should feature at least 10 gallons.

Adjusting the water parameters will also make pregnant zebra danio more likely to spawn. For your convenience, here is the equipment that I use in my tank:

  • I measure the pH, ammonia, nitrates, and nitrites in my tank with the API Aquarium Test Kit (link to Amazon). That bundle allows me to perform hundreds of measures. Within five minutes, I know if something went wrong.

I also recommend adding plants like Java Fern to the aquarium. The fish will appreciate the hiding places. Don’t forget to check the oxygen levels. Oxygen deficiencies will only exacerbate the conditions causing your fish to swell. Install an efficient filtration system that can keep debris out of the water.

2. Performing Regular Water Changes

Water changes are an essential component of a healthy zebra danio aquarium. An efficient filtration system isn’t enough to maintain the integrity of your water. Water changes can combat temperature spikes while also lowering the concentration of toxins and eliminating bacteria and parasites.

Depending on your water source, you may have to apply water conditioners. You can use them to remove ammonia, chlorine, copper, and the like. They can also stabilize the pH, not to mention adding essential minerals.

Regular water changes, accompanied by the use of the relevant water conditioners, will improve your aquarium water quality. This development can alleviate stress, allowing sick, distressed, and bloated fish to recover at a much faster rate.

3. Feeding Your Danio Properly

It isn’t enough to give the zebra danios a balanced diet consisting of flakes, pellets, cucumber, spinach, and the like. You have to avoid overfeeding. Give the creatures food in quantities they can finish in three minutes.[7] Better yet, get an automatic feeder.

I personally use the Eheim Automatic Feeding Unit (link to Amazon). All you have to do is set the time, and you are good to go. The device will also ensure that your fish are fed with equal amounts of food.

If your fish is constipated because you overfed it, place it on a diet consisting of peeled and boiled peas for a few days. You can also treat it to an Epsom salt bath. Once the fish’s constipation clears, the zebra danio’s bloated stomach will shrink.

4. Taking Care of Pregnant Zebra Danios

Mating danios should be placed in breeding tanks. Once they lay their eggs and the male danios fertilize them, move the parents back to the main tank. Otherwise, they will eat their offspring. The females should regain their original shape once they lay their eggs.

5. Dealing With Dropsy

Dropsy kills most of the fish that contract it, which is why many aquarists euthanize fish with Dropsy. However, if you are determined to save your zebra danios, you have a few treatment options at your disposal, including:[8]

  1. Move the zebra danio to a separate tank. This allows you to treat the fish without affecting its tankmates.
  2. Add a teaspoon of salt for every four gallons of water. Salt baths are encouraged because they can remove some of the excess water from the danio, reducing the swelling.
  3. Because bacteria cause Dropsy, the sick fish may respond to antibacterial food. If you don’t have antibacterial food on hand, you can add antibiotics like tetracycline to the danio’s food.
  4. The fish requires high-quality food.
  5. If the danio fails to improve after a few days, apply Maracyn Two to the water. If this step also fails to produce results, talk to a vet. If they cannot identify a more effective treatment, your only option is to euthanize the sick fish.

How to Tell if my Zebra Danio is Pregnant?

These signs indicate that a Zebra Danio is pregnant:

  1. The female danio will become very active.
  2. The fish will gain weight and appear wider than usual.
  3. You will notice a bulge in the midsection of the female’s body.
  4. The water temperature will be around 78 degrees F.
  5. The male danio will swim frantically around the female.

Zebra danios lay eggs. They are not live-bearers. As such, they do not swell quite as dramatically during the mating period. However, there are signs that you can still look for to determine whether or not the fish are pregnant, including:

  • Temperature – This was mentioned before. Zebra danios require a temperature in the 70s (Fahrenheit) to spawn. The ideal temperature is 78 degrees F.[9] But if the temperature is in the low to mid-70s, spawning is just as likely to occur.
  • Gender – If you think that a zebra danio is pregnant, check its gender to ensure that it is female. Only female danios lay eggs. As was also noted before, they are larger and less colorful than male zebra danios.
  • Size – A pregnant female zebra danio is wider than usual.[10] Naturally, you cannot assume that a zebra danio is pregnant simply because it has gained weight. However, if the danio in question is female and the temperature is appropriate, and you have male danios in the same tank, you have every reason to conclude that the bloated fish is pregnant.
  • Behavior – Mating zebra danios tend to chase each other around. Their behavior may imitate violence in some cases, but it is perfectly safe. Zebra danios will chase each other if they are about to spawn, if they are in the process of spawning, or if they have already spawned.

As you have probably noted, you cannot accurately determine whether or not a zebra danio is pregnant by looking at one sign. You have to consider multiple symptoms.

Related articles:


If your zebra danio is fat, you should first consider pregnancy. First, make sure that the fish is indeed a female (you can use the first video in this article). Then, look for additional signs such as frantic swimming and the typical bulge.

I also suggest that you avoid overfeeding. Feed your danios the amount they can finish within two to three minutes. If you notice leftovers floating in your tank, you’ve probably fed the fish too much. Overfeeding may cause constipation and bloat.


  1. https://www.aquariumnexus.com/pregnant-zebra-danio/
  2. https://www.fishtankworld.com/signs-of-overfeeding-fish/
  3. https://thefisheriesblog.com/2018/01/08/fat-fish/
  4. https://nippyfish.net/sick-betta/dropsy/
  5. https://www.hartz.com/how-to-deal-with-an-overweight-fish/
  6. https://www.theaquariumguide.com/articles/zebra-danio-3
  7. https://www.fishkeepingworld.com/zebra-danio/
  8. https://www.ratemyfishtank.com/blog/what-is-dropsy-and-how-do-i-treat-it
  9. https://fluffyplanet.com/how-to-tell-if-your-zebra-danio-is-pregnant/
  10. https://www.cuteness.com/article/danio-fish-pregnant