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Molly Fish Staying at the Top of the Tank: 5 Easy Solutions

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Being an aquarist, I tend to observe my fish quite frequently. That helps me to identify worrying signals that require my attention. For example, I noticed that my molly fish stay at the top of the tank for extended periods quite a few times. Over the years, I’ve learned a few reasons for that phenomenon.

Molly fish typically swim and stay at the top of the tank due to a swim bladder disease, secondary to constipation and overfeeding. The condition disturbs the fish’s buoyancy, forcing it to float. However, mollies also frequent the top sections due to oxygen deficiencies and elevated toxins.

As we proceed in this article, I will list a few other reasons that may have forced your molly to swim upwards. I will also take you step-by-step to solve this issue so that your fish gets the proper treatment and remains healthy.

Why do Mollies Stay at the Top of the Tank?

Mollies are small fish, but they are very active. They need as much room as you can give them to explore. They shouldn’t spend all their time at the top of the tank, and if they are doing just that, one or more of the following is to blame:

1. Your Molly is Suffering From a Swim Bladder Disease

The swim bladder is an organ that allows the molly fish to swim upright and to maintain its desired depth in the water.[1] The molly fish manifests erratic swimming behavior whenever the swim bladder stops working. 

In some cases, it will sink to the bottom of the tank, where it will remain because its ability to swim has been compromised. In other cases, it will rise to the top, where it will also remain because, as was mentioned, its ability to swim has been damaged. 

Swim bladder disease can occur in situations where the other organs push against the swim bladder. It can also occur because of constipation, overfeeding, and diseases. If your molly changed its swimming behavior, you should suspect that its swim bladder is to blame.

2. The Fish is not Getting Enough Oxygen

When a fish starts hanging out at the surface, your first concern should be the oxygen levels in the water. That is because fish use their gills to extract oxygen from the water. Naturally, oxygen enters the tank through the surface.

If the oxygen levels dip drastically, the only rational course of action for the molly is to rush to the surface where the oxygen levels are still sufficient. 

A molly fish hanging out at the top because of an oxygen deficiency will also gasp for breath, rapidly opening and closing its mouth.[2] If the tank’s oxygen levels do not improve, the molly will eventually become lethargic because it cannot find the strength to swim as actively as it once did. 

Some common causes of oxygen deficiency include:

  • Temperature – Water with a high temperature cannot hold as much oxygen as water with a lower temperature. Overheating in the tank can strip the water of its oxygen, causing your fish to look for oxygen at the surface. That typically happens due to the ambient temperature during the summer, a malfunctioning heater, or excessively bright lights.
  • Stagnation – Even when oxygen dissolves into the tank in sufficient amounts, the water must be agitated to ensure that the oxygen is circulated evenly throughout the aquarium. If the water is allowed to stagnate, some sections will develop oxygen deficiencies. Stagnation can occur in tanks that don’t feature filters and pumps.
  • Overstocking – You cannot afford to crowd your tank with mollies. They will use up all the oxygen, causing a deficiency that will force some of them to frequent the top of the tank. That also happens in tanks that hold an excessive amount of vegetation.
  • Waste – Excess waste in the aquarium can reduce the available oxygen. It can do this by clogging the filters (which prevents them from sufficiently agitating the water) and enhancing algae’s growth, which consumes oxygen at night.[3] It is also worth noting that the process by which bacteria break down waste uses oxygen. The more waste your tank has, the more oxygen this process uses.
  • Plants – Live plants are supposed to produce oxygen. People add them to aquariums to boost oxygen levels. However, live plants only make oxygen when you expose them to light. In the darkness, they actually consume oxygen. If you have too many plants and your aquarium doesn’t get enough light, the plants will cause an oxygen deficiency.

3. Your Molly isn’t Happy in its Tank

Sometimes, mollies run to the top and stay there because of an oxygen deficiency. But more often than not, they are doing this to escape the poor conditions below, which include:[4]

  • High Toxins Concentration – The presence of fish in a tank typically attracts ammonia, nitrates, and nitrites. Of the three, ammonia is the most dangerous. It only takes a small amount of ammonia to harm your fish. A higher concentration will eventually kill the mollies. 

But before that happens, they will struggle to breathe. If you study them closely, you will notice that the gills are inflamed. It will also become evident that they are gasping for breath, which typically shows that the tank is oxygen-deficient. 

But if you’ve checked your tank and the oxygen levels are appropriate, you have to consider the ammonia levels. Mollies will run to the surface to escape the ammonia. They will stay at the top until the situation below is resolved.

  • Parameters – Mollies are touted as tough fish that can survive in various conditions. However, they need specific parameters in the water to thrive. That includes the pH and the temperature. While they can tolerate the wrong parameters for a time, the discomfort will eventually become too much for them. 

If you accidentally exposed your mollies to the wrong pH or temperature, they may respond by swimming at the top. Bear in mind that the most troublesome of these parameters is the hardness. Mollies hate soft water.

  • Size – Mollies are active fish that need their freedom. If you force them to inhabit a cramped, overstocked aquarium, they will look for relief at the top, though this doesn’t always help. That is why aquarists usually grow mollies in wide tanks rather than tall ones.

4. The Fish is Hungry

Some fish frequent the aquarium’s surface because they are hungry, and they know that you always feed them from the top. If you only see the mollies in this section of the tank during mealtimes, they have merely adapted to the schedule you have set. 

Mollies know when to expect food. If you can see them at the top during other periods of the day, they are probably hungry, and they hope that they will receive food by hanging out at the top. Typically, they swim back to the lower sections after mealtimes.

5. Your Molly is Stressed

Stress can compel fish to act in ways that are not wholly natural. That includes either lying still at the bottom of the tank or staying at the top. Stress has many sources, including low water conditions, bullies, bright lights, underfeeding, overfeeding, the absence of foliage, human traffic in the vicinity of the aquarium, etc.

6. The Fish is Carrying a Disease

Like stress, an illness or infection can change the way mollies behave. While some of them become lethargic, others will look for relief at the top, especially when parasites have invaded their gills.[5]

If that is indeed the case, your molly will produce symptoms that are similar to those found in oxygen-deficient tanks, such as labored breathing and gasping. You may even notice that the gills have changed their shape and no longer seem healthy.

Dealing with Mollies that Always Swim at the Top

While some mollies can stay at the top because they want to, this isn’t normal behavior, and you can use the following steps to tackle it:

1. Oxygenating Your Tank

There are several ways to increase the oxygen levels in an aquarium, including:[6]

  • Water Change – A massive water change will bring immediate relief to your aquarium, and therefore should be your first step. The new water added to the tank as a result of the water change will increase the oxygen levels. 
  • Agitation – Start by installing powerheads, air stones, and filters. If you already have a filter, get a more robust model. You should also consider installing multiple filters if your tank is too large. These devices will increase the amount of oxygen entering the aquarium and ensure that this oxygen is evenly distributed.

If you are looking for a cheap solution, I highly recommend the hygger Aquarium Air Stone Kit (link to Amazon). That affordable bundle will elevate the oxygen dramatically while maintaining a quiet environment.

  • Temperature – If your tank is overheating, start by removing the lights and deactivating the heater. Leave it off until the temperature in the tank falls significantly. If the water is still too hot, use a fan to blow across the tank’s surface. 

In the case of an emergency, you shouldn’t hesitate to add a few ice cubes (in zip-close bags) to the water. I also suggest keeping a thermometer on hand. You don’t want to lower the temperature so much that the water becomes too cold for your mollies.

In case your heater stopped working correctly, you should definitely check the Cobalt Aquatics Flat Neo-Therm Heater (link to Amazon). After trying dozens of heaters, that is the only device that didn’t disappoint me. It keeps your temperature incredibly stable, as I demonstrated in my review.

  • Maintenance – Try to keep the tank clean as much as you can. Vacuum the substrate on occasion. It is crucial to remove all leftovers and dead organisms that you find. If you permit the waste in the water to accumulate, ammonia and other toxins will rise.
  • Plants – You can increase the oxygen levels in the tank by adding more plants. But before that, you should ensure that the aquarium has sufficient lighting. Otherwise, the plants will consume the oxygen instead of making it.

2. Adjusting the Water for Mollies

Molly tanks require regular water changes. This is the only way to control the concentration of toxins like ammonia. However, water changes are not enough. You need a decent filter that will remove debris from the water.

As was noted above, the substrate should be vacuumed, and dead plants and animals should be removed. Test the pH (7.5 to 8.5), temperature (75 to 80 degrees F), and hardness (12 to 25 dGH, 10-25 dkH) regularly to ensure that they do not stray from the ideal range.[7]

On that matter, I highly recommend checking the API Aquarium Test Kit (link to Amazon). That bundle will accurately measure your pH, ammonia, nitrates, and nitrites and signal if something has gone wrong. That is the particular kit that I’ve been using for over a year with great success.

Also, please avoid overstocking and overcrowding at all costs. The average tank requires one gallon of water for every inch of an adult fish. If your tank is too small, feel free to check my recommendations for aquarium kits.

3. Treating Swim Bladder Disease

It would help if you had a conducive environment in the aquarium to treat swim bladder disease. The wrong parameters can exacerbate the condition. Low temperatures are especially problematic because they cause digestive complications.

Once you improve conditions in the tank, you should place the mollies on a 48-hour fast. This will give them time to digest the food they already have in their bodies. After the fast, use cooked, peeled peas to alleviate constipation. Keep the mollies on this diet for three days.

4. Lowering Stress

You can combat stress by creating a friendly environment in the tank. That means adding decorations and foliage for the mollies to hide behind, eliminating bullies, and removing elements that agitate the fish, such as harsh lighting.

5. Feeding the Fish Properly

You shouldn’t respond to a molly that is staying at the top by feeding it. Mollies should be fed twice a day. Also, they should get the amount of food they can finish in two to three minutes. If you notice leftovers after that period, you’ve fed your fish too much.

In case you already fed your molly and are confident that the amounts you added were sufficient, don’t feed it again simply because it is hanging out at the top. Like most fish, mollies will keep eating if you keep feeding them. This is why overfeeding is so common. 

If your schedule is hectic and you occasionally forget to feed your fish, add an automatic feeder; It will ensure that they are fed on time. I personally use the Eheim Automatic Feeding Unit (link to Amazon). With this device, I don’t have to worry about my fish being starved nor overfed.

If you found this content useful, here are a few related articles that may also interest you:

Conclusions

If your molly regularly swims at the top sections of the tank, constipation and a swim bladder disease should be among your first considerations. With this condition, you will also notice odd swimming behaviors, such as erratic swimming.

While treating that condition, you should also test the water parameters, including pH, temperature, ammonia, nitrates, and nitrites. You can easily do that with a test kit. If one of the parameters is wrong, conduct more frequent water changes and clean the tank properly.

References

  1. https://www.cuteness.com/article/cure-bladder-disease-molly-fish
  2. https://www.thesprucepets.com/low-oxygen-in-aquarium-water-1381215
  3. https://www.lsu.edu/seagrantfish/resources/factsheets/algae.htm
  4. https://www.petco.com/content/petco/PetcoStore/en_US/pet-services/resource-center/health-wellness/why-is-my-fish-breathing-at-the-surface.html
  5. https://www.swelluk.com/blog/why-are-my-fish-swimming-at-the-top-of-the-tank/
  6. https://aquagoodness.com/how-to-increase-oxygen-in-fish-tank/
  7. https://modestfish.com/molly-fish-care/