Why Do Molly Fish Stay At The Bottom Of The Tank?

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Mollies are among my favorite type of fish. They get along with most freshwater fish, and they are pretty easy to take care of. Nevertheless, a few times, they got me worried. I noticed that they spend a lot of time at the bottom of the tank. To solve that issue, I began to research the topic a bit deeper. 

Molly fish occupy the bottom of the tank due to inappropriate temperature or water conditions, including high ammonia and nitrite concentrations. However, that could also be a sign of an underlying ailment, such as Ich and swim bladder disease. 

As we move forward, I will elaborate on a few more reasons for the phenomenon. Then, I will show you what steps you should take to solve the issue. To your surprise, that behavior could be merely a sign of a sleeping molly. 

Why Is My Molly Fish Laying On The Bottom Of The Tank?

Is it normal for molly fish to stay at the bottom of the tank? Well, it isn’t. That is why you should look into the matter if you notice such behavior in your Mollies. Mollies will frequent the bottom of the tank for any one of several reasons, including:

1. Pregnancy

Mollies are livebearers; they give birth to live babies. While it isn’t always the case, pregnant molly fish have been known to frequent the bottom of the tank. They might even stay there for several days at a time.

You can rule out this factor by determining whether or not you have a female molly fish on your hands. Female mollies have an anal fin that looks like a half-circle. In males, this fin is a pointy knife. If your molly fish is male, pregnancy cannot be the cause of such behavior.

On the other hand, if it is female, you can look for additional signs such as a swollen stomach.[1] Pregnant mollies also tend to move slower. They eat more, and they avoid their tank mates. If you study the anal vent, you will notice a dark triangular spot in the vicinity.

Also Read: Pregnant Molly Fish 101

2. Stressful Conditions

Several elements can cause stress in molly fish. That includes poor water conditions, violent tank mates, and new environments. Some fish struggle with disorientation when you first introduce them to a new tank.

Others will react negatively to new lighting, unexpected movements, and changes in decorative items. These reactions will drive them to seek refuge at the bottom of the tank. You should suspect that case if you’ve made some recent changes in your aquarium.

Also Read: Stress In Molly Fish

3. Sleeping Behavior

Some people never consider the possibility that their fish might be sleeping. Admittedly, it isn’t always easy to differentiate between a sleeping fish and merely lethargic and inactive. That is why you observe a bit more carefully.

You should look at the way your molly is hovering. Sick, or even dead fish, will lie on their sides. If your fish is merely sleeping, it will probably remain upright and balanced. If you pay close attention, you will also notice that its fins are slowly moving.

The mouth, on the other hand, will stop moving altogether. You should also consider the time. Fish will sleep an average of 9 hours. Sometimes, that figure rises to 12 hours. This happens at night when the lights have gone off.

Once the lights return and activity breaks out in the aquarium, a healthy molly fish will revive, becoming active once more. One way of determining the health of an inactive fish is to sprinkle some food around it. A sleeping fish will eventually respond. The same cannot be said for a sick or dead fish.

Also Read: Why Is My Molly Not Moving?

4. Exhaustion

Aquarium lights are supposed to create a night/day cycle in your tank. After a while, you will find that your fish only sleep when the lights are off. However, if you leave the lights on, you could prevent your mollies from falling asleep. 

Eventually, they will show signs of exhaustion during the day. That includes sinking to the bottom every so often. That is more likely to be if the aquarium is exposed to lights during the late-night hours.

5. Swim Bladder Disease

Swim bladder disease occurs when the swim bladder’s function is compromised. The swim bladder is crucial to your fish’s ability to remain balanced as it swims in the tank. Factors such as overeating and constipation can cause organs in the molly fish to push against the swim bladder, debilitating its operations.

This can also happen as a result of bacterial and parasitic infections. Swim bladder disease prevents molly fish from swimming conventionally. While some might simply move up and down in the tank, others might either swim erratically or linger at the bottom or top of the aquarium.

6. Inappropriate Temperature

Temperatures that are higher than the ideal range will cause discomfort in your fish. Warm water does not hold as much oxygen as cold water. A spike in temperatures could result in an oxygen deficiency.

This will compel some fish to seek sanctuary at the top of the tank. However, it is also worth noting that a spike in temperature can cause warm water to rise. This creates a layer of cold, oxygenated water at the bottom. Molly fish will seek out this section of the tank to alleviate their distress.

Also Read: Molly Fish Temperature

7. Ich Disease

Ich is a disease that is easy enough to identify because it produces white spots. It is not a comfortable illness. Mollies that contract ich can be seen rubbing their bodies against hard surfaces. This is one reason that might explain your molly fish’s decision to spend so much time at the bottom. 

Rubbing against objects at the bottom, such as rocks and substrate, could relieve the uncomfortable feeling in your mollies.[2] If your molly fish keeps scratching against the substrate at the bottom, Ich is probably the cause.

Also Read: Molly Fish Ich

8. Poor Water Conditions

A tank with inadequate water conditions can send your molly fish to the bottom. Ammonia, nitrates, and nitrites are dangerous to fish. If you don’t have a testing kit on hand, but you want to determine whether the water conditions are appropriate, look at the molly’s gills. 

If they have started leaning towards a reddish or purplish color, ammonia is probably the cause. You should also look for crimson streaks on the fins. Yet, as I will show you, later on, it is much better to have a water testing kit.

How To Prevent Your Mollies From Lying At The Bottom?

The only way to keep your mollies from spending so much time at the bottom is to eliminate the factors causing them to seek shelter down there. You should also treat any ailments that have shaped this behavior in the tank. Some practical options include:

1. Ensure Proper Water Conditions

Try to maintain optimal conditions in your molly tank. That means getting an aquarium of at least ten gallons to avoid overcrowding. You should also keep the temperature between 72 degrees F and 78 degrees F, and the pH between 6.7 and 8.5.[3]

Keep your testing kits on hand. I personally use the API Water Test Kit (link to Amazon). It is reasonably cheap and gets the job done. Generally, you should keep an eye on the temperature, pH, ammonia, nitrate, and nitrite concentrations in the tank. 

If toxins are the cause, replace the water in your tank more frequently; one-third of the tank each week. Also, identify and eliminate components that elevate the concentration of toxins, such as dead plants and rotting leftovers.

Also Read: What Is The Best pH Level For Molly Fish?

2. Get The Right Tankmates

A common source of stress in fish is hostile tankmates. If you don’t want your mollies to hide at the bottom, remove all the other fish in the tank that keep harassing them. If that isn’t an option, move your molly fish to a separate tank.

If that isn’t an option, add some plants and decorations that your mollies can use to hide from aggressive tankmates. Make every effort possible to create a peaceful environment in the tank.

Also Read: 19 Great Neon Tetra Tank Mates

3. Use Your Lighting Properly

Create and maintain a regular day/night cycle for your molly fish. Invest in lighting that can be programmed. This way, you don’t have to worry about inducing exhaustion in your mollies by forgetting to turn their lights off at night. Automatic lights will activate at night and deactivate in the morning regardless of your presence and absence. 

The one that I use is the Current USA 18″-24″ Satellite Plus PRO LED Light (link to Amazon). It is a pretty high-end device, although the remote control, a wide range of options, and timer are undoubtedly worth checking out. It will benefit your tank in the long run.

Also Read: Do Molly Fish Need Light?

4. Filtration

Consider getting a filter if you lack one. It will keep your water clean. You should also carry out regular water changes to keep the ammonia levels at a minimum. Don’t forget to clean your tank. That includes the walls and substrate. This will prevent toxins such as nitrites from building up.

It is worth noting that a powerful filter will enhance the distribution of oxygen by disturbing the water. Hence, you should also consider adding an air stone. Keep an eye on the temperature. Check your heater from time to time to ensure that it hasn’t malfunctioned.

Also Read: Do Molly Fish Need A Filter?

5. Swim Bladder Disease

If your molly fish has swim bladder disease, you should raise the temperature to 80 degrees F.[4]

If constipation is the cause, don’t feed your molly for two days. Give its digestive tract a chance to resolve this issue. Many professionals encourage aquarists to feed their fish peeled peas (as was described in the Youtube video above).

You should also add some aquarium salt to the water. But this is only after placing the molly in a separate recovery tank. If you suspect an infection, use antibiotic food to tackle it. If the molly fails to recover, consult a vet.

To prevent similar complications in the future, avoid overfeeding. By maintaining a pristine tank, you can also avoid illnesses that cause swim bladder disease. The same goes for maintaining a balanced diet.

Also Read: Molly Fish Swim Bladder Disease

Why Do Mollies Stay At The Top Of The Tank?

A molly fish staying at the top of the tank could be just as problematic as one that frequents the bottom. To your surprise, there is an overlap for some of the reasons for both phenomena. Hence, each change in your molly’s swimming behavior should raise your suspicion.

Molly fish stay at the top of the tank for the following reasons:

  1. Swim bladder diseases, mostly caused by an exaggerated diet.
  2. Lack of oxygen concentrations.
  3. Your molly is searching for food, which is usually coming from the top.
  4. High toxins concentrations, such as ammonia and nitrite.
  5. Inappropriate tank size and overcrowded conditions.

1. Swim Bladder Ailment

Swim bladder disease should be your first consideration whenever you notice unusual patterns in a fish’s swimming behavior. A fish that suffers from this condition could swim either upwards or downwards, sometimes even vertically. 

If a disease or injury compromises the fish’s swim bladder, it will have a difficult time swimming traditionally, and it might float to the top. Common causes of swim bladder disease include a poor diet and distended organs.

2. Lack of Oxygen

If your tank has an oxygen deficiency, your fish will swim to the top. You will see them gasping for oxygen at the surface.[5] This is a sure sign that the lower sections of the tank don’t have oxygen in sufficient quantities. Your molly fish will stay at the top so long as the situation below remains unchanged.

You should also suspect oxygen deprivation if your fish is continuously opening and closing its mouth, as I elaborated in this article. Besides oxygen deficiency, I mentioned there a few other reasons that may cause it and a few ways to solve them. 

3. Poor Food Resources

If fish are hungry, they will run to the top of the tank. This is because they know that most of their food comes from above. As such, they will frequent this region in the hopes of being fed. They will stay at the top of the tank so long as their food problem remains unresolved.

That is probably the case if your mollies leave the upper sections after being fed. Then, after a few hours, they may come back to the top sections, looking for the next meal. If that is the case, you may have fed your fish portions that are too small. Try feeding them the amount they consume within two to three minutes, no more and no less.

4. High Ammonia Levels

If your tank has high ammonia concentrations, your mollies will attempt to escape the resulting discomfort by running to the top. The same thing will happen if the concentration of nitrites and nitrates is high.

As was mentioned earlier, you should test your water for toxins, preferably weekly. The test kit is your best option on that matter. You may also see symptoms in your fish’s behavior, although they appear once the condition is about to be too late. 

5. Tank Size

If your tank is small and overstocked, a molly fish might flee to the surface to escape the overcrowding below. Overcrowding is a source of stress, and stress has been known to impact a fish’s behavior in the tank.

To solve this problem, you should either get a larger tank or dilute your fish population. As a rule of thumb, each inch of fish should be accompanied with one gallon of water. For example, two mollies, which are 2 inches long, require four gallons of water. 

Also Read: Why Is My Molly Hiding?


Molly fish could spend their time at the bottom of the tank because they are suffering. That could be the behavior your fish present when they are sick or when the water conditions are inappropriate. 

However, molly fish may also stay low once they are exhausted or merely asleep. These cases naturally resolve once you turn on the lights, or provide your fish with a decent day/night cycle (about 8-10 hours of daylight within 24 hours).


  1. https://www.cuteness.com/article/tell-molly-fish-pregnant
  2. https://www.cuteness.com/article/causes-fish-lie-bottoms-aquariums
  3. https://www.fishkeepingworld.com/molly-fish/
  4. https://www.cuteness.com/article/cure-bladder-disease-molly-fish
  5. https://www.thesprucepets.com/low-oxygen-in-aquarium-water-1381215