Why My Guppy Died After Water Change?

Most people think guppies are sensible fish, incapable of adapting to change and that pretty much everything can kill them. That’s obviously not true since guppies are extremely hardy, resilient, and adaptable. It’s not the change itself that kills them, but the pace at which the change occurs.

In other words, abrupt environmental changes will hurt guppies the most.

This is how we get to today’s hot topic: guppies dying after a water change. If this is your first experience as an aquarium owner, you probably know how important water changes are. But more important than the process itself is how you perform it.

Today, we will dissect the main reasons why guppies tend to die soon after a water change.

6 Reasons Why Guppies Die After Water Change

Guppies don’t do well with sudden changes in their habitat. This includes temperature, chemistry, pH, lighting, and even the tank’s overall layout. This brings us to the 6 most important reasons why your guppies might die following a water change:

– Chlorine in Tap Water

This is the number one cause of guppies dying after a water change. Fortunately, it’s a mistake that only uninformed novice guppy keepers will make. Their mistake consists of thinking that if tap water is safe for humans, it must be safe for guppies. Unfortunately, that’s not the case since tap water often contains vast amounts of chlorine, along with a variety of chloramines.

Water authorities will use chlorine to disinfect the water and make it safe for human consumption. The chemical is harmless to humans, but it’s toxic to guppies, especially when in larger quantities.

You should either not use tap water at all or, at least, use a dechlorinating method prior to using it. The easiest dechlorinating method involves leaving the tap water to ‘breathe’ for at least 24 hours before using it. This will allow chlorine to dissipate naturally.

Or you can boil the water to speed up the process. Just make sure you allow it to cool off before using it. Guppies are not fond of boiled water. Generally speaking.

– Temperature Shock

The water’s temperature may fluctuate dramatically during a water change if you don’t pay attention. Many people will ignore this aspect and generally pour cold water over their guppies. This can send them into shock and kill them instantly.

Guppies can adapt to a significant temperature range, typically between 65 to 85 F. Their ideal living temperature should be somewhere between 72 to 82 F, preferably in the middle 70s. However, dropping the temperature to 65 F, for instance, immediately can kill your guppies. As will keeping them at that temperature for long periods of time.

You should always check the water’s temperature before pouring it in your guppies’ tank. Aside from the risk of causing sudden death, cold water can also lower your guppies’ immune system. This will make them vulnerable to diseases, parasites, and bacterial infections.

Equalize the new water’s temperature with the one in your guppies’ tank to minimize these issues.

– Too Much Water Change

Performing too frequent or large water changes can hurt your aquarium fauna. Any aquatic environment will develop beneficial bacteria cultures that feed off of fish waste and other decaying matter. Too frequent water changes will disrupt these cultures, affecting the environment as a result.

You shouldn’t change more than 10% to 15% of the water on each occasion to prevent this. Water changes of 30% and up risk destroying the beneficial bacteria, creating an imbalance in the environment.

The frequency of the water changes also depends on several factors. These include your aquarium’s size, how many fish you have, how messy they are, how much they eat, etc. I recommend performing at least one water change worth of 15% of the total water volume weekly.

This is typically enough to cleanse your guppies’ habitat and keep the fish healthy and active over the years.

– Tank Was Not Cycled

A massive water change will kill off or remove the beneficial bacteria in the tank, disrupting the habitat’s bio-balance. The first problem you will notice as a result is the dangerous spike in ammonia and nitrites. These are toxic to your fish, and now there’s nothing to prevent these chemicals from spreading.

If the water change is too abrupt, you should at least cycle the tank to ensure the system’s stability. The cycling process typically occurs naturally as the cultures of beneficial bacteria consume ammonia and turn it into nitrites, which are still toxic compounds. New bacteria will then consume the nitrites and turn them into nitrates which are no longer a problem for guppies, except in larger quantities.

However, changing too much of the water will require your intervention to set the cycling process in motion. I’ve written a detailed article on tank cycling that you can check since it’s too much to write for this section.

– Washed Filter Media

The filtering system will harbor much of the beneficial bacteria residing in your tank. Overcleaning or disinfecting the filter media will destroy these bacteria, creating an imbalance in the tank’s bio-film.

You should only clean the filter media with tank water to prevent that. Don’t use tap water due to its chlorine content, and don’t use any cleaning products that might sterilize the component.

– Different Water Parameters

Pouring new water into your fish tank is always dangerous. That danger increases the more water you displace due to the changes in water parameters. These may include fluctuations in temperature, pH, ammonia, nitrates, and excessive amounts of harmful bacteria.

Balancing the water’s parameters is essential for preventing your guppies from experiencing any health problems. You should also note that some signs of disease will only become visible days later, at which point it could be too late.


Water changes are necessary since they balance your guppies’ environment, prevent ammonia buildup, and ensure better water oxygenation. They might also result in more harm than good when performed poorly.

I hope this article can prevent your guppies from dying and teach you how to perform water changes safely and effectively. Don’t worry, you will get better at it with time. The important thing is not sacrificing your guppies’ life during this learning curve.

avatar Noah
I’m Noah, chief editor at VIVO Pets and the proud owner of a playful, energetic husky (Max). I’ve been a volunteer at Rex Animal Rescue for over 2 years. I love learning and writing about different animals that can be kept as pets. read more...

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