Why Are My Mollies Keep Dying?

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The molly fish is a wildly popular species in the aquarium world for good reasons. It is hardy, adaptable, easy to maintain, and friendly and acceptant of other species.

Mollies make for the perfect choice for novice fish keepers since they can thrive in a variety of environments, including freshwater, brackish water, rivers, and even in the ocean for a while.

However, sometimes, even the durable molly can get into trouble. This species is usually acceptant of some variations in their water parameters, so long as they’re not sudden, massive, or too frequent.

This being said, they can die at times, and figuring out why fast is vital.

Mollies are not a schooling species, but they do perform better when kept in groups of at least 6 individuals.

This means that, in most cases, there will be several mollies in the same tank. If one of them dies, it’s imperative to figure out the cause, otherwise, others might start dying as well.

But why do mollies die? Here are the 9 most common reasons for that:

1. Water Quality

This issue is most commonly the result of 3 other problems:

  • Lacking a tank filter – The primary purpose of a filter is to house cultures of billions of microorganisms that will consume ammonia and nitrites and turn them into nitrates. Ammonia is deadly in high enough concentrations, and different fish species are more sensitive than others. The filter will also remove fish waste, food residues, and dirty floating in the tank, keeping the water cleaner and minimizing the need for too frequent tank maintenance. Lacking a filter will cause the water quality to degrade faster.
  • Lacking tank maintenance – Your molly tank still requires occasional maintenance to remove excess fish waste buried in the substrate, vacuum food residues, and remove dead plant and animal matter. It’s also a good opportunity to eliminate excess algae deposits which may degrade the water’s quality. Unfortunately, many people skip this phase completely or perform tank maintenance rarely, when absolutely necessary. At which point it may already be far too late.
  • Lacking water changesChanging the water is also a must for mollies and other fish species alike. The procedure will dilute the excess ammonia, cleanse the water, oxygenate the fish’s environment, and re-mineralize their habitat. You may need to perform water changes of 10-15% weekly, depending on how many mollies you have, how large the tank is, and how dirty it gets within those 7 days.

The problem with the degradation in water quality is that it can turn deadly fast. To fix the issue, consider the previous 3 points.

Get a reliable filter, perform regular tank maintenance and substrate vacuuming, and change the water regularly.

These won’t take much time but will impact your mollies positively in the long run and prevent sudden, unexplained deaths along the way.

2. Ammonia Poisoning

If your mollies experience ammonia poisoning, the problem is already more advanced than it should’ve been.

Ammonia poisoning doesn’t happen suddenly. It’s a gradual process that stems from the accumulation of toxins and harmful residues in your fish tank.

There are several issues that will lead to ammonia poisoning, including:

  • OverstockingKeeping a lot of mollies in an unjustly small environment will cause problems fast. Among the most obvious ones is the excess fish waste that will accumulate on the substrate, boosting ammonia levels and poisoning the water. Another problem relates to fish stress since mollies will become more aggressive in tight spaces and when overcrowded. Fish stress will lower your mollies immune system, which, when combined with elevated ammonia, can only spell disaster.
  • Large tanks, many fish, no maintenance – This doesn’t relate to overstocking, but rather to having a lot of fish in a large tank with no maintenance. This will make it difficult to monitor the fish population, so if one fish dies, you may not notice. The fish will decay out of sight, its body poisoning the water and boosting ammonia levels fast. This will lead to even more losses of life among your mollies.
  • Lack of water changes – I agree that regularly changing your fish’s water can be a chore, but it’s also necessary. Especially if you don’t have a filter. Performing water changes every week or even every other week is key to maintaining a healthy and stable environment for your mollies. Otherwise, the harmful bacteria will take over, boosting the ammonia levels and increasing the risk of ammonia poisoning.

But what are the symptoms of ammonia poisoning?

The most relevant signs include your fish gasping for air, displaying erratic swimming, and showcasing red or even bloody gills.

In advanced stages, your mollies may even show falling scales and skin ulcerations prior to coma and death, which are soon to follow.

The solution is simple. Have a filter, provide the tank with a lot of plants, perform regular maintenance and water changes, and never overstock the fish.

As a side note, you should always monitor the ammonia levels to ensure they remain at the optimal value, 0.

3. Chlorine Poisoning

This is another common problem that’s more widespread among novice molly keepers.

The issue will most notably occur as a result of improper water changes or filter cleaning, although the former can have more devastating effects. In short, using tap water during water changes can kill your mollies.

Regular tap water contains chlorine, along with other chloramines meant to disinfect the water and make it drinkable for humans.

Chlorine is harmless to humans, but it acts as a poison to most tank fish and other aquatic creatures.

You don’t need much to kill your mollies, which goes to show how devastating a 10-15% water change involving tap water can be.

The same issue occurs when cleaning the filter with chlorinated tap water.

Aside from chlorine killing the beneficial microorganisms inhabiting the filter, it will also affect the fish if you mount the filter into the aquarium soon after cleaning it.

The solution comes in the form of 3 options:

  • RO Water – RO stands for Reverse Osmosis, which is the process of eliminating as many water particles as possible, including various minerals and harmful microorganisms. It is great for your aquarium, provided you re-mineralize it to provide your mollies with a more nutritious aquatic habitat.
  • Deionized Water – The deionization process relies on resin-based substances to trap electrically charged harmful particles and replace them with hydrogen ions. The purpose is to sterilize the water somewhat, making it safer for fish. Just like RO water, deionized water requires re-mineralization before adding it to your tank.
  • De-chlorinated Water – Yes, you can use tap water for your water changes and filter maintenance, so long as you de-chlorinate it first. The process is as easy as allowing the water to ‘breathe’ for 24-48 hours before using it. The chlorine will dissipate naturally during that time. To accelerate the process, you can also boil it, which will eliminate chlorine a lot faster. Or use de-chlorinating solutions with the same goal.

Whichever option you choose, always monitor the water’s chlorine levels before using it to make sure it’s safe.

4. Temperature Shock

This is another issue more strictly related to water changes. Mollies prefer a water range between 72 and 78 F, which are tropical conditions.

They can withstand some variations in water temperature, but never too sudden, too massive, or too frequent. If the water grows too cold or drops too low in temperature, your mollies may experience temperature shock.

The symptoms include lethargy, hiding behavior, lack of erratic swimming, and death. Although death isn’t really a symptom, it’s the cessation of all symptoms.

To prevent temperature shock, always adjust the water’s temperature during a water change before pouring it into the main tank.

The goal is to balance the water’s temperature with that of the tank water to minimize the risk of temperature shock.

Not doing so will, at best, cause the fish some physical stress and, at worst, kill them.

None of these are desirable outcomes.

5. Aggressive Tank Mates

Pairing mollies with aggressive and territorial fish species is the worst thing to happen to mollies since ammonia poisoning.

Cichlids, especially, have a knack for pushing mollies to their limits. Mollies are peaceful and friendly fish, especially when in groups of at least 6 individuals, with male mollies only displaying moderate territorial behavior.

Other larger or more aggressive fish can attack and bully mollies, which can force them into hiding.

The physical violence itself may not harm mollies directly, but it will stress them out. The constant stress will affect your mollies’ immune system, rendering them vulnerable to health issues like infections, parasites, or various fish diseases.

To prevent this problem, only pair mollies with other peaceful fish like tetras, guppies, swordtails, or platies, to name a few.

You should also rely on plants and aquatic decorations to make the aquatic habitat look more natural.

Mollies, along with other fish, will use these elements to hide and rest during more stressful times, which could involve territorial fighting, bullying, playing, or female or food competition.

6. Disease or Parasites

There are no fish species that are immune to diseases or parasites. If left untreated, these health issues can turn deadly fast. Mollies are vulnerable to health problems like:

  • Ich (White spots)
  • Velvet disease
  • Fin rot
  • Protozoan disease
  • Mouth fungus
  • Dropsy
  • Swollen gills
  • Red-blood spot
  • Viral Hemorrhagic Septicemia
  • Swim bladder disease
  • Popeye disease
  • Gill flukes
  • Camallanus parasites
  • Fish tuberculosis
  • Bent spine
  • Hexamitiasis

While these conditions seem overwhelming, they are not specific to mollies-only. These are common health issues among tank fish, each related to various genetic or environmental factors.

And each comes with a variety of symptoms, although some will display symptoms later than others.

There are 2 solutions that work for all these problems, no matter the condition’s nature:

  • Prevention – Know which disorder occurs under which factors and try to avoid those factors. Keep your mollies’ water clean, avoid feeding food from shady sources, and perform regular water changes and tank maintenance.
  • Identify and treat the conditions fast – Not all these health issues are deadly, but most are, especially when adequate treatment doesn’t come in time. Even more worrying is that several of these disorders are contagious and will quickly spread to healthy fish. Identify the conditions in time and quarantine the sick fish immediately. This will allow you to provide adequate medication and treatment to the sick molly while protecting the rest of the population.


7. Overfeeding

Overfeeding is an issue more common among novice keepers than anyone else.

Beginners tend to think that providing their fish with more food equals better care, but that’s not always the case.

In fact, in most situations, it can deliver results opposite to what’s expected.

3 major problems relate to fish overfeeding:

  • Stress – Your mollies will experience more stress due to constipation and other digestive issues related to overfeeding. And we already know the long-term effects of fish stress.
  • Hepatic lipidosis – This condition is commonly known as fatty liver disease and can turn deadly in some cases. Fortunately, it’s not that common among mollies, but you should never rule it out.
  • Higher ammonia levels – Overfeeding causes more uneaten food, resulting in more food residues decaying in the water. The result is an aggressive shift in the water’s chemistry, with more ammonia being produced as a result.

In the end, overfeeding is never beneficial or even neutral. It always has harmful effects, whether immediate or delayed.

The solution is simple. Try never to overfeed your fish. Only feed your mollies once or twice per day and only enough food for them to consume within a minute.

As a plus, you should vacuum the substrate once a week or every 2 weeks to prevent food decay.

You can even throw in several bristlenose pleco or Corydoras into the mix. These bottom-dwelling catfish will clean up any food residues, keeping the tank water cleaner for longer.

They also make for peaceful tank companions, capable of cohabiting with mollies with little-to-no problems.

8. Poor Genetics

The bent spine syndrome is common among tank fish, including mollies. It is often the result of genetic defects, but it can also trigger as a result of scoliosis or fish tuberculosis.

In other words, your mollies can display the syndrome at birth, or they can develop it over time as adults.

The condition is untreatable and will lead to a painful existence, a shorter lifespan, and even sudden death in most cases.

Euthanasia is necessary to relieve the fish from its suffering.

Another genetic-related issue links to inherited poor immune systems. This will render the fish more vulnerable to any conditions, including bacterial infections, parasites, and other disorders.

Not only that, but these health problems will even be more aggressive in immune-deficient mollies.

There’s little you can do in this sense. The best solution I can provide is being thorough about where you’re getting your mollies.

Only buy them from specialized shops or even private breeders who can account for their fish’s genetic prowess.

9. New Tank Syndrome

To better understand the concept of new tank syndrome, you must first understand nitrifying bacteria.

This is an overarching family of beneficial microorganisms divided into 2 groups: Nitrosomonas and Nitrobacter.

The first group will consume ammonia and turn it into nitrites, while the second group will consume nitrates and turn them into nitrates.

Then you remove nitrates via regular water changes, but this is for another topic.

These bacteria are essential to any aquatic environment. The new tank syndrome always occurs in environments lacking nitrifying bacteria.

But how does that happen?

Let’s look at the 3 most common situations where nitrifying bacteria are absent from the environment:

  • Skipping the nitrogen cycle – This is a new-tank issue (new tank syndrome, get it) occurring when mostly novice fish keepers introduce the fish to their new tank without performing the nitrogen cycle. The nitrogen cycle aims to create the ideal environment for beneficial bacteria to form, and it involves several distinct phases. The cycle can last around 6 to 7 weeks, and it’s vital for crafting a healthy and stable environment for your fish. Without the nitrogen cycle, the tank won’t have any nitrifying bacteria to regulate ammonia, quickly leading to ammonia poisoning.
  • Overcleaning the substrate and the filter – The nitrifying bacteria cultures usually form in the substrate and in the tank’s filter. Vacuuming and cleaning the substrate and the filter too thoroughly and too often will disrupt the tank’s biofilm, killing the nitrifying bacteria and creating the perfect setting for the new tank syndrome.
  • Using antibiotics – Antibiotics are necessary to combat various bacterial infections and fish conditions that could prove deadly otherwise. The problem occurs when using antibiotics in the main tank since these medications don’t discriminate. They are designed to kill all bacteria, whether good or bad, and the nitrifying microorganisms are right in their line of action. If you need to use antibiotics, always quarantine the sick fish first and place it in a treatment tank. You should also use the antibiotics according to an expert’s recommendation.


As you can see, there are a lot of issues that could lead to your mollies’ death.

They may seem overwhelming but are easily countered by simple strategies like changing the water regularly, performing regular maintenance, and providing mollies with optimal living conditions.

I suggest learning about mollies, their preferred water conditions, best tank mates, and behavior to detect and address early problems.

This, combined with a bit of discipline and consideration for your mollies, will keep your fish healthy and thriving over the years to come.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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