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Why Is My Molly Swimming Upside Down? (6 Quick Solutions)

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I remember how worried I got when I saw my first upside-down molly fish. Indeed, I told myself, there has to be something wrong. Luckily, over the years, I learned what causes that issue and how to deal with it. Now, I am willing to share my knowledge with you!

Molly fish tend to swim upside down due to a problem in their swim bladder, also known as swim bladder disease. It is a condition where the fish’s ability to control its buoyancy is impaired, and they are unable to keep themselves upright in the water. Most commonly, it is caused by an infection.

As we proceed, I will discuss how the swim bladder is supposed to work normally, what causes the problem in the first place and what you can do about it.

Why Is My Molly Swimming Upside Down?

A fish’s swim bladder is a gas-filled organ in its body that allows it to control its buoyancy. It is used to maintain a constant neutral buoyancy, and it can be seen as an evolutionarily advanced lung that makes them efficient at living on land.

There are many different causes of swim bladder disease:

1. Bacterial Infection

One of the most common causes of a swim bladder disease is a bacterial infection.[1] The bacteria attach to the swim bladder and cause the fish to start swimming upside down. This is a prevalent infection, especially in freshwater aquariums, and can result from poor water sanitation, highly stressful factors, exposure to chemicals, or even bad water quality.

Other symptoms of a bacterial infection in molly fish include gasping, swimming up and down, and a bulge in the abdomen. The bulge (overflowing swim bladder) is much easier to identify if the fish is swimming upside down. 

The fish is often in distress and maybe swimming frantically as if trying to escape. If your fish is swimming upside down, it is likely suffering from a bacterial infection and needs immediate attention. It needs to be treated with antibiotics as soon as possible.

2. Parasite Infection

Parasites can also cause swim bladder disease, but they typically affect tropical freshwater fish like molly fish prone to bacteria and bacterial infections. Parasites like flukes can attach to the swim bladder and cause the molly fish to begin swimming upside down from then on. 

This is another common cause of swim bladder disease, but it is usually seen as a side effect of another infection. It is more commonly found in freshwater aquariums than saltwater because parasites are much more common in freshwater.

3. Physical Damage

Molly fish can also develop swim bladder disease if they experience physical damage to their swim bladder. This can happen during shipping or if you accidentally hit them while feeding them. 

Physical damage happens when a sharp object, like a claw or fin, gets caught in the swim bladder and causes an injury. The fish will be unable to right itself if the injury is severe enough. If your fish is swimming upside down, you should check it for any physical damage to the swim bladder.

4. Fluctuating Water pH

Another largely unknown reason that molly fish tend to swim upside down is fluctuating pH levels. Whether you live in a tropical or temperate climate, the pH level in your water fluctuates throughout the year. 

When the ph levels rise and fall sharply, it can cause the swim bladder to malfunction. The molly fish will begin swimming upside down until it acclimates to its new pH level. Molly fish, in general, are known for their quick ability to adapt and adjust to new conditions.

Yet, in an environment where pH levels fluctuate, this can be a problem. To fix swim bladder disease in your molly fish, you need to provide a stable ph level environment. You should perform regular partial water changes or carbon changeovers every 4-7 days.

5. Constipation

A constipation problem is another cause of swim bladder disease.[2] Constipation occurs when a fish’s meal becomes too hard and requires extensive time to pass through the digestive tract. In freshwater aquariums, this can be due to feeding too much at one time or feeding too large a meal. 

Furthermore, molly fish often eat only small amounts of food at a time which can lead to constipation. The fish eats, digests the food for a few days, then stops eating. The food expands in its digestive tract and becomes hard enough to cause swim bladder disease symptoms.

You can tell that your molly fish is constipated by looking at its stomach. If the stomach looks damaged or swollen, that is a sign that your fish is constipated. The fish’s abdomen should be round and firm, and you should not be able to see through it.

6. Malnutrition

A lack of nutrients in the diet of your molly fish can cause swim bladder disease. Molly fish that are not getting enough calcium and vitamin D in their diets will be more likely to develop swim bladder disease.  Both of these nutrients are essential to the development of the molly’s swim bladder.

You can tell that your molly is malnourished by looking at the color of its body. If it has a faded, yellowish or pale body color, this can be a sign that it is not getting enough nutrients in its diet. The molly fish will also have difficulty swimming and float upwards.

Mollies are omnivorous, meaning they eat both plants and animals. Because of their omnivorous diet, they tend to require extra nutrition in their diet to sustain themselves.

7. Congenital Defects

Another cause of swim bladder disease in molly fish is congenital disabilities. Mollies are prone to developing swim bladder disease at birth because their swim bladder is not fully developed. A newborn molly is less likely to develop swim bladder disease if its mother was properly conditioned during pregnancy. 

Congenital defects occur when a molly fish’s swim bladder is damaged or not fully developed. The condition is fatal and usually happens during the birthing process. This causes some of the swim bladder to become non-functional, and the molly fish cannot right itself at all.

However, if your molly has just recently developed upside-down swimming, it is unlikely that it had a congenital defect. Congenital defects more commonly occur when the fish is born or at least very young.

Also Read: Molly Fish Swim Bladder Disease

How Do You Treat Swim Bladder Disease In Mollies?

If your molly swims upside down, I highly suggest that you take action to treat its swim bladder disease. Here is what you should do:

1. Treating Bacterial Infections

The best way to treat swim bladder disease caused by a bacterial infection is to use an antibiotic. If you cannot get your fish to a fish store or vet right away, you can use aquarium salt or the API MELAFIX to kill the bacteria in its swim bladder.

The pH level of the water should be at least 6.8 for these antibiotics to work properly. You should also perform daily water changes of at least 15% every day until the bacteria clears up.

In situations like that, I personally use the API AQUARIUM SALT (link to Amazon). Add 1 or 2 tablespoons to every five gallons of water. I do not recommend using aquarium salt unless you know your fish has a bacterial infection. This is because aquarium salt will kill off beneficial bacteria in your molly’s digestive tract, causing more harm than good.

If you use the API MELAFIX (link to Amazon), add 5 ml per 10 gallons of aquarium water. You can also mix some MELAFIX with your aquarium water and add it to the container of aquarium salt.

Also Read: Why Is My Molly Fish Floating At The Top?

2. Treating Parasites in Mollies

To treat parasites, I suggest you use a parasite medication of some sort. If a medically prescribed medication is available, then that is fine. If not, you can use homeopathic or natural remedies, which effectively treat parasites in fish.

However, from my experience, it is best to consult an aquatic veterinarian. You should also do your research to determine what type of parasite you are dealing with. Once diagnosed, you can take a medication that will kill specific parasites and prevent other types from entering the body.

3. Adjusting the Water Parameters

The temperature should be from 75 to 80 degrees Fahrenheit for mollies, and the pH should be from 7.5 to 8.5.[3] Keeping these water parameters is essential to the health of your molly. If they are off, you can’t expect your molly to be healthy. 

The fish’s body will try to compensate for the uncertainties you aren’t aware of, resulting in irregular behavior and illness. Therefore, ensure that these parameters are adjusted as needed throughout the year with or without a thermometer.

To monitor the pH, nitrates, ammonia, and nitrites in my tank, I use the API Aquarium Test Kit (link to Amazon). That bundle quickly measures these parameters, and you can get an easy reading within 30 seconds. It also lasts for hundreds of measures.

To prevent pH fluctuations, you’ll need to have water changes every two or three weeks at around half of the tank’s volume. For example, if your water changes are 50%, then maintain that ratio until the end of your cycle.

4. Treating Physically Damaged Mollies

The best way to treat a physically injured molly is by ensuring that the water parameters are suitable for mollies (as described above). Keeping the parameters within the desired range will help your molly heal on its own. 

However, if the molly still seems to be in pain or it’s bleeding excessively, you can use the API AQUARIUM SALT (link to Amazon) to help treat it. You should put one teaspoon for every five gallons of water. The salt will help with circulation and reduce swelling. 

Mollies are very sensitive to medications, so it is best to consult an aquatic veterinarian if your molly suffers from internal organ damage or swallowing issues. The veterinarian may use a chemical intervention.

5. Helping a Constipated Molly

If you suspect that your molly swims upside down due to constipation, I suggest that you let them fast for three days before feeding. Even though this may sound difficult, it will help you prevent harm to your molly by removing the water from the digestive tract.

If your molly fasts for a couple of days, you will notice that its stomach shrinks and that it becomes easier to treat the constipation problem. Your molly will be able to swallow food without having a hard time and be more comfortable as well.

You can also use peeled peas to provide fiber to your fish.[4] Whole peas are a good source of fiber and natural salts. First, you’ll need to partially cook the peas so that they become soft enough for your molly to eat. 

Then, add the soft peas to some aquarium water in a jar or container and mix it well until the peas dissolve. Feeding your molly this mixture can help them get rid of constipation in no time. 

6. Feeding Your Mollies Right

The best diet for mollies is a variety of freeze-dried and fresh food. You can feed your molly bloodworms, brine shrimp, certain plants (such as Anacharis or water lettuce), algae, daphnia, and other foods that are appropriate for mollies.[5]

You should not just feed your mollies flakes or pellets because it is not a complete diet for them. Mollies need to eat other foods in addition to pellets to keep their bodies healthy.

I also suggest that you use an automatic feeder. This will help your mollies eat at the same time and not overeat. I personally use the Eheim Automatic Feeding Unit (link to Amazon). That device ensures that my fish are fed with the right amounts each day, so it is less likely they will get constipated.

Also Read: Molly Fish Swimming Vertically


If you are one of the many pet owners who has noticed your molly constantly swimming upside down, you might not have any idea of what may be the problem. Several causes affect this behavior in mollies, and it is best to know what they are so that you can get to the root of the problem.

As I mentioned in the beginning, these reasons are just a few known causes, so your molly may have another reason for swimming upside down. But the reasons I mentioned above are the most prevalent and should be treated first before looking elsewhere.