Why do My Mollies Keep Dying?

As small-sized fish that exhibit a lot of variety, mollies are a beloved tropical fish species that can live in brackish water as well.

Molly fish are often chosen by beginners because of the hardiness and unpretentious nature of these fish.

Since they’re often classified as hardy fish adaptable to a range of water conditions, many first-time aquarists make the mistake of not paying enough attention to keeping conditions.

This often causes a ripple effect of disbalances in water conditions, which leads to illnesses and deaths that seemingly come out of nowhere.


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So, if you find yourself wondering “Why do my mollies keep dying?”, below you can read a list I compiled about the possible causes behind molly deaths in your aquarium.

Reasons Why Mollies are Dying

Here are some of the reasons why your molly fish are dying:

1.  Bad Water Quality

Poor water conditions is the leading cause of fish deaths in the aquarium.

Whether it’s the accumulation of too many toxins or changes in water parameters, bad water quality is correlated with diseases and death among molly fish.

Low levels of oxygen in the water, water with temperature outside the accepted range for mollies, and untreated tap water used for water changes can all cause problems in your aquarium.

2.  Aquarium Cycling Issues

If you’ve added molly fish to a newly set up aquarium and after a week your mollies are dying one by one, the problem is that you don’t have a cycled aquarium.

The aquarium cycle or nitrogen cycle is a step you shouldn’t overlook when putting together an aquarium for your fish.

Cycling is required to establish a healthy colony of bacteria that will help keep toxin levels (ammonia and nitrites) in check by transforming them into less harmful substances (nitrates).

Even if you’ve attempted to cycle your aquarium, the cycling process may have failed, which happens if it’s your first time cycling the aquarium.

The nitrogen cycle takes a couple of weeks and many aquarists lose patience waiting it out. In the hope that things will turn out well anyway, they add the fish too early before the cycle is finished.

This is a big mistake, even if mollies are advertised as hardy fish, they’re still sensitive to high toxins. Therefore, the only humane and correct way to cycle the aquarium is without having any fish in the tank before the nitrogen cycle has finished.

In a nutshell, the nitrogen cycle has the following steps:

  1. Add dechlorinated and treated tap water to the aquarium;
  2. Start adding small amounts of fish flake as you would if you’d be feeding your fish daily;
  3. Monitor ammonia levels and follow levels as they spike;
  4. Continue adding small amounts of fish flake to the water;
  5. When ammonia levels are high, start monitoring for nitrites too;
  6. Follow ammonia levels drop and nitrite levels spike as ammonia-eating bacteria start developing;
  7. Wait for ammonia and nitrite levels drop to zero and nitrates appearing as nitrite-eating bacteria develop;
  8. The nitrogen cycle is considered completed once ammonia and nitrites as at 0 ppm, while nitrates are below 20 ppm.

Once the cycle is completed, you can start adding your molly fish one by one to the tank. Monitor toxin levels and perform weekly water changes to dilute toxins and add fresh water to the tank.

3.  High Ammonia Levels

Even with the nitrogen cycle performed, sudden spikes of toxin levels can be observed, especially if your fish produce a lot of waste as a result of overfeeding and overstocking.

Ideally, there should be 0 ppm ammonia in your aquarium, however, ammonia can spike in the following cases:

  • Decomposing fish food that hasn’t been removed from the tank;
  • Dead fish left in the tank to decompose;
  • Dirty substrate and filter.

Therefore, cleaning the substrate and rinsing the filter as well as removing any uneaten food can keep ammonia levels in check.

If the nitrogen cycle has been correctly performed and tank maintenance is carried out on the regular, then there should be no ammonia in the tank.

4.  Aquarium Stocking Issues

As I mentioned in the previous point, overstocking the aquarium is also a potential cause of rising toxin levels in the aquarium.

Even if you believe you have a spacious tank and you’ve added only a few mollies to the tank, in a matter of a single month, you may experience overstocking issues due to the fact that mollies breed at a fast rate and produce a lot of fry.

In this context, you should strive to control their population, starting with adding either only male mollies to your tank or adding only one male to three females.

If you have too many fish (even if they’re just fry) in the aquarium, oxygen levels can drop dramatically, which can be fatal to your mollies.

5.  Overfeeding

Molly fish absolutely love eating and seem insatiable, so it can be difficult to resist the urge of giving them more food than they actually need.

Overfeeding causes mollies to produce a lot of waste, more than you’re removing with weekly water changes.

With waste products accumulating and oxygen levels dropping, fish become ill and eventually die off.

6.  Water Temperature Issues

Mollies do well in temperatures in the range of 70 °F to 82 °F. Even so, sudden changes are not well tolerated and should be avoided.

A water heater is a sure-fire way to avoid spikes or drops in water temperature, so consider investing in a quality heater for your molly fish tank.

7.  Poor Genetics

Because genetics plays an important role in fish longevity, sourcing your mollies from a trusted breeder that breeds healthy fish is crucial.

Large pet stores aren’t known to be the most reliable source for high-quality fish with a healthy genetic profile.

Fish bought from a pet store may be cheaper, but I advise you to pay a bit more extra and source your fish from a breeder, so you can enjoy the company of your mollies for far longer.

8.  Diseases & Parasites

Common fish diseases and parasite caused infections can also cause unwanted deaths in the tank. Diseases can break out because of poor tank conditions, but they can also be introduced via food, other fish, and live plants.

Ich, fin rot, flukes, and velvet disease are just some of the molly fish diseases that can cause early death in your fish if they’re left untreated.

Research the diseases that can affect mollies and the remedies for these diseases, so you can spot, treat and prevent them from killing your fish.

How to Keep Mollies Healthy?

Focusing on raising healthy fish and preventing diseases is the best way to prolong the life of your mollies.

Here’s how to keep your molly fish healthy:

1.  Balanced Diet

A balanced, nutrient-rich diet is one of the bases of promoting a strong immune system and healthy growth and development in your fish.

Molly fish thrive on an omnivorous diet that’s varied and contains both meat-based and plant-based fish foods.

2.  Frequent Water Changes

Don’t get lazy with water changes. Depending on the size of the tank and the number of fish in your stock, perform 20-30% water changes weekly.

Test for toxin levels and use clean, chlorine and heavy metal-free water to replenish the aquarium. Always match the water parameters of the home aquarium.

3.  Tank Maintenance

Cleaning the water alone is not enough, you should take time to clean the substrate, decorations, the inner surfaces of the aquarium and rinse the filters at least monthly.

Skipping on maintenance can allow unwanted debris to accumulate, which can cause ammonia spikes and even algae blooms to spiral out of control.

Never use chlorinated water to clean plants, decorations or filters because you’ll destroy the healthy bacteria that have colonized these things and you’ll harm your fish in the process.

4.  Disease Prevention and/or Early Intervention

Disease prevention is also crucial in keeping molly fish healthy. While not all diseases are preventable, most can be avoided or treated if you act fast.

Be careful when you introduce new fish to the tank. Quarantine them first and monitor for diseases. Only then introduce them to the tank.

Clean plants and decorations that you may want to add to the tank, since these too can carry illnesses and parasites.

Learn about molly fish diseases and remedies, so you can spot problems early on and administer treatment as soon as possible.

If you notice signs of distress in your fish or sign of illnesses, act facts and seek out treatment before it’s too late.

Remove sick or dead fish from the aquarium to prevent the spread of diseases and consider tank level treatments too.


With the proper care, you can enjoy the company of your molly fish without experiencing any problems.

Unfortunately, the eco-balance in the aquarium can be easily disturbed and this can open the way for many problems that can result in the death of your fish.

I hope my article serves as a good basis for further research on how to cycle your aquarium, how to care for molly fish, and molly fish diseases and remedies, aspect that are indispensable knowledge for anyone keeping mollies.

Questions and Answers

Martin bajjada May 7, 2023 Reply

I am martin from malta. At the moment i sm hzving some problem. I have one meter aquarium with lot of guppies and mollies fry. Every day i am finding one or two mollies fry dead. They come skinny then some of them swim in circle then dies. But only mollies. What is the problem and what i can give as a treatment. Ph is 7.8. Thanks

    It could be a diseases that only affects the mollies, or it can be a genetical problem. Also, check for ammonia and nitrites, these can also cause sudden death in fish. If you are feeding too much, it could be a problem of ammonia spikes, make regular water changes of 20-25% in your tank.

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