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Bloating in Fish

Bloating in Fish

Updated on February 03, 2023
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Reviewed by Dr. Jennifer Coates
DVM

Signs of Bloat | Causes | Malawi Bloat | Dropsy | Treatment | Prevention

Bloated Fish

One of the most unpleasant experiences for an aquarist is to peer into an aquarium and notice a fish struggling with bloat. What causes bloating in fish? How can it be treated? Is there any way to prevent fish bloating? 

Bloat—also sometimes called dropsy or Malawi bloat—is almost always a symptom of a primary illness rather than a disease in its own right. It can be difficult to cure, but pet parents have some treatment options if they catch the bloat early enough. Ideally, instead of treating a sick fish, you’ll want to take steps to maintain your fish’s health and wellness. 

Telltale signs of bloat 

As the name implies, the primary symptom of bloat is the swelling of a fish’s body. Typically, this bloating occurs relatively rapidly, which is how aquarists can distinguish fish with bloat from big-belly fish that might just be overweight, constipated or carrying eggs. Bloat is often also accompanied by other symptoms. 

  • Difficulty swimming 
  • Pale gills 
  • Bulging eyes 
  • Lack of appetite 
  • Red patches on the body or fins 
  • Curved spine 

As the condition progresses, bloat can turn into “classic dropsy.” At this stage, the condition has worsened to the point that the fish’s scales protrude away from their body, giving them somewhat of a pine cone look. 

What causes bloated fish?  

Bloat occurs when a fish’s kidneys or gills can’t remove water from their body, which accumulates making the fish’s belly swollen. 

As previously noted, bloat is almost always a symptom of an underlying condition. The primary conditions that most often lead to bloat in fish are: 

  • Bacterial infection 
  • Fungal infection 
  • Viral infection
  • Parasites 
  • Tumors

Many of these conditions can be exacerbated by stress from poor water quality, diet and aggressive tank mates. Chronic stress can degrade your fish’s immune system, making them more vulnerable to disease and parasites.  

What is Malawi bloat? 

Malawi bloat is often lumped together with generic bloat, but there are slight differences. Malawi bloat primarily—though not always—affects African cichlids. These fish are herbivores and mostly eat aquatic plants and algae. Malawi bloat most often occurs when cichlids are fed too much protein, such as bloodworms and brine shrimp.  

One way to treat Malawi bloat is to offer less protein and more plants and algae to African cichlids. Read your fish food labels carefully and choose food formulated specifically for herbivores.  

What is dropsy? 

The terms “fish bloat” and “dropsy” are often used interchangeably, but classic dropsy refers to the last stage of bloat. At this point, a fish becomes so bloated and inundated with water that their scales protrude from their body. Sadly, a fish’s kidneys may be damaged beyond repair by this stage,, which is why it’s so important for bloat in fish to be recognized and treated early. For more information about dropsy, check out our guide to Common Diseases in Aquarium Fish. 

How to treat bloat in fish 

The first step in treating bloat in fish is to quarantine your sick pet from the rest of the group. If possible, set up a hospital aquarium where you can isolate and treat the fish with bloat. Next, perform a 50% water change to help reduce any dirty water from further stressing your fish. Consider adding a dechlorinator and a small amount of aquarium salt to the water.  

You’ll also want to minimize the stress of your sick fish. Dim the lights in the habitat or keep them off. Try to limit noise and vibration in the area around the aquarium. You can also try improving symptoms by not feeding your infected pets. Fish with bloat often have a distressed digestive system and will likely not eat, anyway. Putting food into the aquarium will only dirty the water.  

Reducing stress and cleaning the water in your fish’s habitat can help, but ultimately, you’ll need to treat the underlying cause of the bloat. If possible, seek out an aquatic veterinarian in your area who can diagnose the underlying problem and recommend more specific treatment. 

It can be extremely difficult for pet parents to determine what exactly is causing bloated fish. It may be worth adding an antibiotic, like erythromycin, to your water. Erythromycin can treat a variety of bacterial infections. If it doesn’t seem to be working, you may want to try other antibiotics, like doxycycline, neomycin, tetracycline or ciprofloxacin. However, dumping multiple antibiotics into your fish’s tank is likely to do more harm than good, particularly if a bacterial infection isn’t the problem.

It’s not always easy for aquarists to get their hands on appropriate medication for fish. Some states do not allow the over-the-counter sale of certain fish medicines. You may need to research the laws in your state or work with a veterinarian in your state to access medication for your sick fish. At Petco, we offer non-medicinal solutions that might be able to help, including fish fungus and bacteria control supplies and fish parasite control solutions.  

If you add medicine to your hospital aquarium, remove the activated carbon in your filter so it won’t remove the treatment from the water.  

Finally, it’s important to keep your expectations in check when you notice a fish with a swollen belly. Treating and curing advanced bloat or dropsy can be very difficult. This is why it is so important to prevent bloat from occurring in the first place and to seek out the services of a fish veterinarian as quickly as possible if it does develop.  

How to prevent bloated fish

An important way to prevent fish bloat is to first make sure that its contagious causes don’t enter your pets’ habitat. It’s always a good idea to quarantine new fish for two to four weeks before adding them to your aquarium. During this time, carefully observe the newcomers and ensure that they don’t show any signs of illness or injury. 

Here are a few more ways to help prevent this condition. 

Keep your water clean

Dirty water can be a breeding ground for bacteria and fungi. It can also increase the stress on your fish, making them more susceptible to illness. Perform weekly or biweekly water changes of 25% and test the aquarium water regularly to make sure it’s within healthy parameters for: 

  • Ammonia 
  • Nitrites 
  • Nitrates 
  • pH 
  • Hardness 
  • Alkalinity 

It’s also a good idea to regularly replace your filter and vacuum the waste from the bottom of your aquarium using a gravel vacuum. 

Avoid overcrowding

An overcrowded aquarium can seriously stress your fish and trigger more aggressive behavior. A crowded habitat can also throw off the nitrogen cycle, producing too much waste for your filter to handle. Many beginning aquarists underestimate how much space their fish need to thrive. 

Add friendly aquarium mates

Make sure the fish you add to your aquarium can live well together. That goes beyond just making sure everyone can play nice—you’ll need to ensure that all your fish can thrive in roughly the same water conditions and that each species receives adequate nutrition.  

Feed your fish appropriately

Pick out a high-quality, species-appropriate fish food, and don’t overfeed. As previously discussed, certain fish—like African cichlids—can quickly become ill if fed a protein-based diet.

Keep a hospital aquarium ready

If you notice a big belly fish in your aquarium, time is of the essence. You don’t want to waste time buying a second aquarium or preparing the water for your sick pet. Having a hospital aquarium ready to go will allow you to quickly isolate your sick fish and begin treating them before the bloat turns into classic dropsy. 

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