Why Are My Mollies Dying (and how to stop them!)

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Why do my mollies keep dying? There are 3 main reasons mollies die unexpectedly, they are poor water quality, low oxygen levels, and diseases. Another reason mollies die suddenly is low or high water temperature following a heater failure.

Mollies were some of the first fish I ever kept and bred. Around 30 years ago I started fish keeping. I am now lucky enough to have a dedicated fish room with dozens of tanks, including some with Black Mollies, Sailfin Mollies, and Dalmation Mollies. Mollies are one of my favorite fish.

There is nothing more frustrating than your mollies just dropping dead for no apparent reason. You haven’t done anything differently, but you suddenly start losing fish.

Main Reasons Mollies Die Suddenly:

  • Poor water quality
  • Ammonia spike
  • Disease
  • Stress
  • Oxygen Levels
  • Water temperature
  • Aquarium cycling issues
  • Overstocking the aquarium
  • Overfeeding
  • Poor genetics

Poor Water Quality

Poor water quality may well be the number one reason mollies die suddenly in the aquarium. In nature, water quality rarely changes. Fish evolve to live in their local water and they spend their entire lives with the water remaining more or less constant.

In the aquarium, the fish are totally dependent on us as aquarium owners to look after their water and make sure the quality remains stable. In nature, when fish live in rivers, the water is constantly moving.

The fish stays in one area, but the water passes by all the time. In the aquarium, the water doesn’t go anywhere unless we drain some out and refill it. In lakes, the water doesn’t move as much, but the number of fish in the lake compared to the number of fish in an aquarium is totally different. In our aquariums, we often cram in as many fish as we can.

The main way water quality becomes affected is when the mollies waste can’t be processed quickly enough by the filter. Every time you put food into your molly tank, the fish eat it and then excrete waste.

That waste is actually toxic to fish, so we use a filter to ‘clean’ the water. Bacteria in the filter break the waste down from ammonia to nitrite, then nitrate. The graphic below from Wikipedia shows the nitrogen cycle in a simple diagram.

How Do You Know Your Aquarium Water Quality Is Poor?

To the untrained eye, there is no clear indication your water quality has become poor. The best way to tell is to regularly test your water using an aquarium water test kit. A test kit will give you an instant result and for most kits, you can test for Ammonia, Nitrite, Nitrate, pH, and sometimes chlorine levels.

It is strongly recommended you test your aquarium water on a weekly or fortnightly basis. That way, as your water quality starts to drop, you can catch it early and take steps to rectify the issue.

I have always had good success using the Master Test Kit which is made by API. It is easy to use, quick, and reasonably priced.

API FRESHWATER MASTER TEST KIT 800-Test Freshwater Aquarium Water Master Test Kit, White, Single, Multi-colored
  • Contains one (1) API FRESHWATER MASTER TEST KIT 800-Test Freshwater Aquarium Water Master Test Kit, including 7 bottles of testing solutions, 1 color card and 4 tubes with cap
  • Helps monitor water quality and prevent invisible water problems that can be harmful to fish and cause fish loss
  • Accurately monitors 5 most vital water parameters levels in freshwater aquariums: pH, high range pH, ammonia, nitrite, nitrate
  • Designed for use in freshwater aquariums only
  • Use for weekly monitoring and when water or fish problems appear

Last update on 2024-07-21 / Affiliate links / Images from Amazon Product Advertising API

How To Fix Poor Water Quality In A Molly Tank.

If your water quality has deteriorated to levels where the health of your mollies has become affected, you need to take action. The simplest way to fix poor water quality is to change some of the water in your aquarium.

Drain out somewhere between 25% and 50% of the tank water and replace it with fresh water from the tap (treated with a suitable dechlorinator if required). After a couple of days, retest your water. If the quality is still poor, repeat the water change process.

Ammonia Spike

An ammonia spike is a common cause of sudden deaths in your molly aquarium. Ammonia is essentially fish waste. After the fish eat, they excrete the waste, which is high in ammonia. Ammonia is very toxic to fish.

The most common cause of an ammonia spike is overfeeding of the fish. This can occur when a friend or neighbor is feeding whilst you are on vacation or, if like me, you accidentally tip too much food into the aquarium when the lid falls off!

Another common cause of an ammonia spike is when a fish dies, but you don’t notice. The dead fish gets stuck behind a rock or decoration and starts to decompose. As it does, the ammonia levels in the aquarium can rise rapidly, leading to more fish dying. The ammonia spike can have a domino effect, killing more fish, leading to more ammonia in the water.

The best solution to an ammonia spike is changing water. As with poor water quality generally, changing 25% to 50% of your aquarium water will bring the ammonia levels down rapidly. A day or two after the water change, retest the water for ammonia and, if necessary, change another 25% to 50% of the water.


Diseases can be introduced to your aquarium via new fish, new plants, and feeding live foods. Identifying the disease is the first step to treating the fish. Does the fish have white spots or fungus, are their gills moving rapidly, is the fish struggling to stay upright.

Look for the obvious signs of disease and then contact your local fish store. Explain the issues you are having to the employee at the local store and ask them to recommend an appropriate treatment.


Stress is a massive killer of fish. Fish can become stressed when water quality deteriorates or the water temperature drops too low.

Fish can also become stressed when physical conditions are causing them problems. For example, male mollies can harass females when the males want to breed. If you keep 4 males to 1 female, the female can literally be harassed to death.

Other fish who like a cave to hide in, but are kept in a bare aquarium can become so stressed they simply die.

External factors like children banging on the aquarium glass, or excess loud noises can also stress your mollies.


Even though they live in water, mollies still need oxygen to survive. The difference between us and them is they have to draw their oxygen from the water. When oxygen levels get low in an aquarium, fish can be seen gasping at the surface.

Factors that affect oxygen levels include overstocking, water quality, and water temperature.

If your mollies are gasping at the surface, it is because there isn’t enough dissolved oxygen in their water. To fix the problem, first, carry out a 25% to 50% water change.

Next, double-check the temperature of the water. If the temperature is too high, check your aquarium heater isn’t stuck in the ‘on’ position.

Finally, think about adding an aquarium air pump with an air stone attached. The bubbles coming out of the airstone will agitate the water surface, increasing the amount of oxygen available to the mollies.

Water Temperature

Mollies like the water temperature to be somewhere between 75°F and 80°F. Temperatures outside of this range can lead to stress in your mollies which in turn may lead to your mollies becoming susceptible to diseases.

The main ways we get temperature issues in our molly aquariums are when an aquarium heater fails and stops heating the water or gets stuck in the ‘on’ position and essentially cooks your mollies.

It is difficult to prevent aquarium heaters from failing. Even expensive, good-quality heaters fail occasionally. The best way to guard against a heater failure is to have a good quality digital thermometer in your aquarium.

Check the thermometer reading every time you interact with your aquarium. Hopefully that way you will spot any issues before your mollies start to die.

Aquarium Cycling Issues

New aquariums have to be ‘cycled’ before the fish can be added. Aquarium cycling is a major topic in the fishkeeping world. The subject is too large to address in this particular article, so below I have included a video that explains the process and why it is so important to the health and survival of your fish.

Overstocking An Aquarium

A molly aquarium can become overcrowded very quickly. If you have 1 male and 4 females, you have the potential for 100 to 400 babies to be added to the aquarium every 6 to 8 weeks. When we keep live-bearing fish like mollies, we have to keep control of the population either by keeping a fish that will prey on some of the molly babies or by having homes lined up for all your molly offspring.

Overcrowding can also happen following a visit to your local fish store. You go in for some food and before you know it you are walking out with an amazing school of this fish or pair of that fish. Tank space is finite, so try not to keep adding fish.

If you have recently added fish and then your mollies started dying, overcrowding could be an issue


Overfeeding can be a real issue for your mollies. I know for me, feeding my mollies is my favorite part of the hobby. Because feeding is so enjoyable, we run the risk of overdoing it. If we put too much fish food into the tank in one go, the fish may not eat all of it. Any food that isn’t eaten will usually fall to the substrate, down behind plants, or into crevices between rocks.

This uneaten food will then start to break down and as it does, releases ammonia. As we saw above, this ammonia can build up quickly in the aquarium and kill your mollies before you realize there is a problem.

To prevent issues from overfeeding, only add about as much food to the aquarium as your mollies will eat within about 2 or 3 minutes. If the mollies eat all the food quickly, add a little bit more. Repeat until your mollies lose interest in the food.

Poor Genetics

Unfortunately, being a livebearer and relatively cheap to buy, sometimes mollies that are sold in pet stores are just genetically weak.

No matter how hard you try and how much effort you put into getting your aquarium perfect for your mollies, they just aren’t strong enough to survive.

To avoid buying mollies with weak genes, only buy from a reputable store. When you are in the fish store, spend some time watching the fish. Check they are lively and moving freely. Never make the mistake of falling in love with the weakfish hiding in the corner.

Even if you take it home and provide a perfect environment for it, it may just die anyway.

How To Keep Your Mollies Healthy

Now we know what some of the issues can be that cause your mollies to die unexpectedly, let’s look at some ways we can make sure we keep them healthy and thriving.

  • Regular water changes
  • Feeding a balanced diet
  • Maintaining the aquarium
  • Disease prevention and treatment

Regular Water Changes

As discussed above, fish are stuck in the water we provide them. They have no chance to get away when water quality slips. To prevent your mollies from dying due to poor water quality, try to get into a routine of changing water on a regular basis.

Draining your tank down by 25% each week and topping up with fresh, dechlorinated water is undoubtedly one of the best things you can do for your mollies. Water changes not only reduce nitrates in the water but by interacting with your tank every week, there is a greater chance you will spot issues before they become major problems.

Feeding A Balanced Diet

Mollies are omnivores. They need a wide variety of foods. For the biggest, strongest, healthiest mollies, you need to feed them a balanced diet.

A good mix of dried foods, live foods, and frozen foods will help give your mollies all the vitamins and minerals they need to be active and colorful.

My article What do Black Mollies Eat? has good information that applies to all members of the Molly family.

Maintaining The Aquarium

Along with regular water changes, keeping up with your general aquarium maintenance will go a long way to stopping your mollies from dying unexpectedly. Cleaning your filters, vacuuming the gravel, and trimming plants are all jobs that help keep an aquarium system running and your mollies thriving.

Preventing And Treating Disease

Prevention is always better than cure when it comes to fish diseases. The number one way disease makes it into your aquarium is by being introduced through new fish.

Without any doubt, quarantining your new fish is a must. .

If you don’t yet have a quarantine tank, this video shows how to make a basic one without spending a fortune.

In Conclusion

Mollies shouldn’t just die for no reason. They are generally hardy fish and you should expect them to live for 3 to 5 years in your care. Taking time to get the basics right will go a long way to stopping your mollies from dying.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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