Why Are My Guppies Dying? (and how to stop them?)

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Why are my guppies dying? There are three main reasons guppies can suddenly start to die. They are poor water quality, low oxygen levels, and diseases. Another major reason guppies die suddenly is due to high or low water temperatures following a heater failure.

I first started keeping guppies over 30 years ago when I was given a 10-gallon tank with a small group of guppies. I don’t think a single day has passed since then when I didn’t have at least one aquarium in my fish room filled with guppies.

It can be incredibly frustrating when your guppies appear to die for no reason. You haven’t done anything differently before, but they just seem to be dying.

Main Reasons Guppies Die Suddenly

  • Water Temperature
  • Poor Water Quality
  • Ammonia Spike
  • Disease
  • Aquarium Cycling Issues
  • Aquarium Overstocking
  • Overfeeding
  • Poor genetics
  • Stress
  • Oxygen Levels

Check Your Guppies Water Temperature

Guppies are hardy fish and they can live in water with temperatures ranging from 75°F to 82°F. If guppies are kept in water that is cooler or warmer than these temperatures, the stress can lead to an onset of disease.

The number one cause of temperature issues in a guppy tank is a heater failing. If the heater stops working, the water temperature will drop rapidly. Occasionally a heater can get stuck in the ‘on’ position meaning it slowly cooks your guppies in the aquarium. Even expensive, good-quality heaters can fail sometimes.

The best way to check if your aquarium heater is working properly is to use a good-quality digital thermometer. If you can see the display on the thermometer every time you feed the fish or service the tank, there is a good chance you will notice a heater failure before your guppies start to die.

Test Your Guppies Water Quality

Poor water quality probably leads to more guppy deaths than any other cause. Guppies have evolved to live in flowing streams where the water they live in is constantly changing.

In the aquarium, the guppies are stuck with whatever water we provide for them. An aquarium full of guppies, eating and pooping the whole time will quickly end up with poor water quality unless we keep on top of it.

The main way your guppy aquarium water quality deteriorates is when your aquarium filter can’t ‘clean’ the water quickly enough. Every time you put food into your guppy aquarium they eat it, then excrete waste.

A guppies waste is high in ammonia and ammonia is toxic to fish. Bacteria in the guppies aquarium filter breaks down the ammonia firstly into nitrite, which is still toxic to fish, then again into nitrate, which is less harmful to guppies, providing levels of nitrates stay low.

The infographic below from Wikipedia shows the nitrogen cycle in an aquarium.

When a filter can’t process the guppy’s waste, the levels of ammonia can build to what is known as an ammonia spike. Sometimes, as the filter processes the ammonia, it can’t process the nitrites, so you get a nitrite spike, although this is less common.

Finally, as the filter processes the nitrites, you get nitrates. Nitrates are less toxic to guppies and build up slowly over time. When the level of nitrates gets too high, your guppies can start to die.

The number one way to reduce the nitrate levels in your aquarium is to perform a water change. By changing 25% to 50% of the water in your aquarium, you export those nitrates and thereby reduce the levels in the aquarium.

Below is a video from Aquarium Co-op that explains the importance of regular water changes in your guppy aquarium


Ammonia Spike

As discussed above, an ammonia spike is caused when the waste from your fish can’t be processed quickly enough by the filter on your guppies aquarium. Ammonia is very toxic to fish.

There are 3 main causes of ammonia spikes in the guppy aquarium.

  • Overfeeding: Feeding your guppies is probably the most enjoyable part of the hobby. However, It is easy to overfeed your guppies. They just come to the surface begging for food constantly. The problem with overfeeding is, any food which doesn’t get eaten, sinks down to the substrate or goes behind the plants or maybe falls into the crevices between rocks. The food then sits there and rots. This rotting food releases ammonia into the aquarium water, potentially leading to an ammonia spike.

  • Dead fish: If a fish dies, and we don’t notice, the decaying fish releases ammonia into the aquarium. This ammonia can rise rapidly which can lead to an ammonia spike. The ammonia spiking can cause other fish to die, thereby having a domino effect of fish dying, releasing ammonia causing more fish to die.

  • Adding new fish: Adding new fish to the aquarium, increases the amount of waste the filter has to cope with. If we have an aquarium with 10 fish in it, then you go to the store and buy a school of another 10 fish, you effectively double the amount of waste being pumped into the aquarium, without giving your filter a chance to double the amount of bacteria available to break down that waste. Increasing the number of fish in the aquarium should always be done slowly.

If you find you have an ammonia spike, the best course of action is to change 25% to 50% of the water. Drain the water out and replace it with fresh water from the tap (dechlorinated if necessary).

After a couple of days, test your water for ammonia. If the levels are still high, change another 25% to 50%. Repeat this process until your ammonia levels read zero on an ammonia test kit.


Diseases are another major cause of unexpected guppy deaths in an aquarium. Diseases can be introduced by new fish, new plants, or feeding live foods to your guppies. Identifying a disease that is affecting your guppies is the priority. Disease identification can be tricky.

Spend some time looking at your guppies. Make notes of what symptoms you can see (for example, white spots, laying on its side, gasping at the surface) and contact your local fish store.

When you explain the issues your guppies are having they should be able to recommend a suitable product.

Check out my articles Why Does My Guppy Have White Spots? or Why is my Guppy Swollen? for more information about common guppy diseases.

Aquarium Cycling Issues

Aquariums need to be ‘cycled’ before fish are added to them. Aquarium cycling is a hot topic in aquarium keeping at the moment. The topic is probably too large to tackle in an article like this, so below I have included a video by KG Tropicals explaining cycling a new aquarium.

Aquarium Overstocking

It is easy for a guppy aquarium to become overcrowded. Just 4 female guppies cranking out 50 babies each a month can lead to a tank full in no time. Whenever we keep livebearing fish like guppies, we need a plan for the babies.

Lining up good homes ready can be one way to keep the number of babies down, as can having a mildly aggressive tank mate like an angelfish that will eat a few of the weaker babies.

Overcrowding can also happen following a visit to the local fish store. We all try and cram as many fish into a small space as possible. You pop into the store for some fish food and walk out with 10 new fish.

Adding those 10 fish to the tank can be enough to tip the balance over the edge and leave you with a tank that has more fish in it than the filter can accommodate.


Overfeeding can really be a problem in a guppy tank. All those little fish bobbing at the surface begging for some more fish food. Feeding is one of the really enjoyable parts of owning an aquarium. The problem is, if you constantly put too much food in, the uneaten food sinks to the bottom of the aquarium, where it sits and rot, pushing up ammonia levels in the aquarium.

Another problem related to overfeeding is when you go on vacation and leave a friend or family member to feed your fish. If they are not fishkeepers themselves I can almost guarantee they will put too much food in.

To prevent issues with overfeeding, only put in as much food as your guppies will consume within a couple of minutes. If they eat it all quickly, put another small pinch in. Repeat, adding a little at a time, until the guppies lose interest in the food.

Poor Genetics

Being one of the world’s most popular pet fish, and being so easy to breed, guppies are bred in their millions every year. Unfortunately, a large number of guppies are from poor-quality stock. They are just genetically weak.

When you buy new guppies, only buy them from a reputable store. When in the store, take a few minutes to watch the tank. Check all the fish are lively and not just sitting on the bottom of the aquarium. Never make the mistake of trying to ‘save’ the sick-looking one in the corner of the aquarium.

If you are breeding your own guppies to increase your numbers, try and only allow the strongest fish to breed. If you allow a weak or mutated fish to breed, their offspring may well be weak and mutated. In nature, these fish wouldn’t survive, and they probably won’t live too long in your aquarium either.


Stress can be a massive killer of guppies. Guppies can be stressed for any number of reasons. If the heater has failed and the tank is too cold, the stress can kill a guppy. Another common way stress kills guppies is when males harass females to death. If you have 3 or 4 males to every female, between them the males will literally pester that female to death.

External factors like children hitting the aquarium glass or loud noise from a stereo system vibrating through the water every day can also cause stress to set in and kill your guppies.

Oxygen Levels

Even though they live in water, fish 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 guppies are gasping at the surface, 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 guppies.

The video below by KG Tropicals discusses whether or not an aquarium needs an air pump.

How To Keep Your Guppies Healthy

So we have had a look at some of the issues that can make your guppies die unexpectedly. Now, let’s look at some of the basics we can do to keep our guppies healthy

  • Feeding a balanced diet
  • Properly maintaining the aquarium
  • Regular water changes
  • Preventing diseases

Feeding A Balanced Diet

Guppies are omnivores. They will eat just about anything we put in their aquariums. Just because they will eat anything, doesn’t mean they should. It is important to give them a variety of foods including tropical fish flake food, pellet food, and some live or frozen foods.

A good mix of foods will give your guppies the proper mix of vitamins and minerals they need to be bright, colorful, and healthy.

Properly Maintaining The Aquarium

Regular aquarium maintenance will go a long way to keeping your guppies happy and healthy. Vacuuming the aquarium gravel, cleaning your guppy’s aquarium filter, and removing dead and dying plants will all help keep your guppy’s aquarium water clean and safe for your guppies.

Regular Water Changes

If there is one secret to a successful guppy aquarium, water changes are it. Changing 25% to 50% of your aquarium water once a week will lower nitrate levels, remove excess fish hormones and, combined with gravel cleaning, remove physical detritus from your guppy aquarium.

Preventing Diseases

If you can prevent diseases from getting into your aquarium in the first place, you reduce the risks of your guppies dying from a disease.

Diseases are generally introduced via two main methods, 1, by introducing new fish, and 2, by feeding poor quality live foods. Solving these problems can be simple.

Before you introduce new fish, consider quarantining them for at least a week, but 2 or 3 weeks is better. By quarantining your new fish you have the chance to see if they develop any diseases or harbor any pests.

You can also take some preventative measures such as treating internal worms. When it comes to living foods, only feed your guppies live foods from reputable sources.

Never feed live foods collected from the wild as you don’t know if they are carrying any diseases.

If you don’t have a quarantine tank yet, check out the video below where you will learn how to make a quarantine tank without spending a fortune.

In Conclusion

Guppies are generally hardy fish. They don’t just start dying for no apparent reason. If your guppies are dying suddenly, work through the list above step by step and you should be able to prevent any further deaths.

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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