Molly Fish Swimming Vertically: 5 Essential Solutions

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Fish cannot speak; however, they do present behaviors that indicate that something isn’t right. For example, quite a few times, I caught my molly fish swimming vertically. Over the years, I investigated the issue and learned how to fix it. Now, I am willing to share my experience.

Molly fish tend to swim vertically when they are stressed. That usually happens due to a disease involving the swim bladder, an organ responsible for the fish’s buoyancy. However, mollies will also swim up and down when they are pregnant, bullied, or when the aquarium conditions are inadequate.

As we proceed, I will share five crucial steps to treat a molly fish that consistently swims vertically. That includes checking the water parameters using the API Aquarium Test Kit (link to Amazon) to hasten the healing process of a swim bladder disease.

Why Is My Molly Swimming Vertically?

Molly fish shouldn’t swim vertically. This behavior isn’t normal. If you have noticed this trend in your molly fish, consider the following potential causes.

1. Swim Bladder Disease

The swim bladder is a vital organ that affects a fish’s ability to swim. Under normal circumstances, it controls the creature’s buoyancy. When the swim bladder functions are compromised, the molly’s ability to swim will suffer in some way.

Anything that compresses the swim bladder can harm it in the long run. That includes constipation and overfeeding. It would help if you also kept an eye out for infections, bacterial and parasitic, and ailments that cause the abdominal organs to enlarge. Some fish are born with defective swim bladders.

Because the swim bladder affects the fish’s ability to swim, a malfunctioning swim bladder will change how the molly swims. Some fish will sink to the bottom because they cannot maintain their buoyancy. Others will float to the top. Vertical swimming isn’t necessarily typical. However, it isn’t unheard of in fish with swim bladder disease.

The symptoms of swim bladder disease are not limited to erratic swimming. It would be best if you also looked for a curved back, bloated stomach, and a poor appetite.[1] Mollies whose swim bladder disease was caused by constipation may stop eating altogether, either because they don’t want to or cannot swim to the food.

Also Read: Molly Fish Swim Bladder Disease

2. Your Molly is Pregnant

Swim bladder disease is the first cause people think about when they notice that their mollies are swimming vertically. Few aquarists realize that this behavior might be a symptom of pregnancy. This is what you should know:[2]

  • Mollies are livebearers. They give birth to live fish.
  • You need both a male and a female molly for breeding to occur. The male molly must fertilize the eggs in the female molly’s body.
  • Male mollies have larger fins, bolder color patterns, and a gonopodium that they use for fertilization.
  • Male mollies are ready to reproduce at twelve months. Female mollies are mature enough to reproduce at six months.

Mollies are quite enthusiastic where mating is concerned. In fact, they are so passionate that you need to keep two to three female mollies in a tank for every male molly. Otherwise, the males will not only fight one another for the females’ attention, but they may harass the females to death if their numbers are insufficient.

Because mollies are eager breeders, you don’t have to coerce them to mate. If you have both genders in your aquarium, you will find that the females are almost always pregnant. When mollies want to mate, they will chase one another. Sometimes, this behavior looks like aggression. 

But it shouldn’t worry you, not unless the male mollies are causing physical harm to their female counterparts. A pregnant molly can be typically identified by its enlarged abdomen, gravid spot, lethargic attitude, and anti-social behavior.[3]

Also Read: Why Is My Molly Fat?

When a pregnant molly is about to give birth, it will swim vertically. You will also observe some shimmying. Suppose you have a female molly in the tank, and you have observed all the signs associated with mating and pregnancy. In that case, you have every reason to assume that it is only swimming vertically because it is about to give birth.

Some people dismiss pregnancy as a potential cause because they don’t have male mollies in their tank. However, they should bear in mind that female molly fish can store sperm for several months. 

They can use that stored sperm to fall pregnant in the absence of a male mating partner. In other words, even in a tank with only female mollies, vertical swimming can point to pregnancy. That is especially true if the fish in question has a distended belly and a gravid spot.

3. The Molly Fish is Stressed

Once you rule out pregnancy and swim bladder disease, the only other significant factor that might explain vertical swimming is stress. Stress is directly or indirectly responsible for most of the bizarre behavior you find in aquariums. It has numerous sources, including:

  • Tank Conditions – What kind of conditions are your mollies inhabiting? Is the water too hard or too soft? How about the temperature? Have you permitted the water to cool beyond the appropriate parameters? Or maybe your heater has malfunctioned, and the temperature has skyrocketed. 

And what about the toxins? Is the aquarium cycled? Has the concentration of ammonia exceeded safe levels? Your molly’s health is directly tied to the conditions in its aquatic environment. If the tank’s conditions deteriorate, you will notice various stress responses among your fish, including vertical swimming.

  • Food – Where food is concerned, overfeeding is just as dangerous as underfeeding. Mollies cannot survive without food. However, if you give them too much food, you will make them sick. Overfeeding can also kill them. Along with attracting toxins like ammonia, overfeeding is associated with ailments such as constipation.
  • Tankmates – Mollies are peaceful fish that will thrive in a community tank, but only if their neighbors are equally peaceful. A community tank with large and aggressive fish will make your mollies miserable. This will produce the sort of stress that may manifest symptoms such as vertical swimming.

How to Treat Molly Fish That Swim Vertically?

If your mollies are swimming vertically, you should customize their treatments to fit the cause, for instance:

1. Treating Swim Bladder Disease

While swim bladder disease is a severe ailment, it has relatively straightforward solutions, including:[4]

  • Temperature – Raise the temperature to 80 degrees F. Keep the temperature at this level during the molly’s treatment.
  • Fasting – If constipation is responsible for the disease, stop feeding the molly. Allow it to fast for the next three days. The molly needs time to digest the food it already has in the digestive tract. Do not lower the temperature.
  • Peas – Peas (cooked and skinned) are one of the most effective answers to constipation. Feed the molly two to three times a day. Please do not give it any other food item.
  • Drugs – If an infection caused the disease, talk to your vet. They will recommend an antibiotic. That is especially true if your molly presents additional symptoms, such as discoloration and lethargy.
  • Salt – Salt helps with infections. It will kill the bacteria that caused the disease. You need one tablespoon for every five gallons. I personally use the API Aquarium Salt (link to Amazon).

Keep the water clean. If you permit the conditions in the tank to deteriorate, the disease will get even worse. Once your molly’s regular feeding schedule resumes, maintain a balanced diet that has high-quality food items.

2. Adjusting the Tank Conditions

You cannot prevent your mollies from swimming vertically without maintaining the appropriate conditions in their aquarium. Keep an eye on the following:

  • Parameters – Your aquarium should have a pH of 7.5 to 8.5, water hardness of 10 to 25 dH, and a temperature of 70 to 82 degrees F.[5]

That is where I usually recommend getting the API Aquarium Test Kit (link to Amazon). That highly affordable bundle allows you to measure the pH, ammonia, nitrates, and nitrites in your aquarium. Within five minutes, you’ll know if something went wrong.

  • Tank Size – You need a tank ranging from 15 to 30 gallons minimum, depending on the type of molly. Mollies hate overcrowding. If you force them to share a small tank with many fish, they will manifest signs of stress, such as vertical swimming. You can save yourself from this headache by getting the biggest tank possible.
  • Air stones – Add airstones to the tank to improve oxygenation. The lack of oxygen can quickly stress your mollies, forcing them to swim vertically as they gasp for air. I personally use the Hygger Aquarium Air Stone Kit (link to Amazon), which does a fantastic job in my aquarium.
  • Plants – I highly recommend adding a few plants and decorations to your tank. That will create hiding places for your mollies and alleviate their stress, especially if their tank has aggressive fish.

3. Replacing the Water Regularly

Mollies require regular water changes. You should also apply conditioners whenever you add water from a source with toxic elements like chlorine. You can easily do that by mixing tap water with the well-known API TAP Water Conditioner (link to Amazon).

Water changes keep the water clean and prevent the concentration of toxins from rising. You should also invest in a decent filter. Filters and water changes work hand in hand to prevent waste from overwhelming your tank.

4. Picking the Right Tankmates

Your mollies cannot escape the stress responsible for their vertical swimming without a peaceful environment. You should remove any aggressive creatures in the aquarium that bully them. Surround them with suitable species like danios, tetras, and zebra loaches.[6] If you have multiple mollies, you need at least two female mollies for every male molly.

5. Feeding Your Mollies Properly

Please feed your mollies well. They will eat everything from zucchini, green beans, and spirulina to blackworms, flakes, and pellets. Please don’t feed them more than thrice a day. Give them food they can consume in three minutes. 

As I mentioned earlier, overfeeding is a considerable risk factor for constipation and swim bladder disease. For that reason, I highly recommend considering the Eheim Automatic Feeding Unit (link to Amazon). By using that device, I make sure that my fish enjoy equal meal portions regularly.

How to Tell if a Molly Fish is Dying?

These signs indicate that a molly fish is dying:

  1. A dying molly fish won’t eat as it used to.
  2. Dying mollies will present discoloration and will usually appear bright and pale.
  3. The fish will find it difficult to breathe, and as a consequence, will lie at the bottom of the tank.
  4. Dying mollies are very inactive; some of them won’t swim at all. Others will continue to swim, but they will do so lethargically.
  5. A sick molly may become anti-social, not only avoiding the other fish but also choosing to stay out of sight.

Molly fish that swim vertically aren’t necessarily dying. However, that could be an early sign of stress which can get worse in the future. For that reason, I highly suggest taking the necessary steps to solve the issue beforehand.

Also Read: Why Is My Molly Swimming Upside Down?


If your molly is swimming vertically, it requires your attention. That is typically a sign that the fish is stressed and will possibly deteriorate in the future. Start by checking the water parameters, including the pH, ammonia, and temperature.

If you suspect that your molly caught a swim bladder disease, isolate it in a hospital tank and follow the recommendations above. Bear in mind that pregnancy is another possible reason for the issue. In that case, the molly will swell and present the typically gravid spot at the lower part of its posterior abdomen.