Why Goldfish Is Dying After Water Change?

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As a cold-water exotic fish, the goldfish ranks among the most adaptable and beautiful tank species you can find.

This species requires pretty much no introduction, so here are a few fast facts to lay the foundation:

  • The ideal water temperature rests between 68 and 74 °F for most species, although some can go as low as 60-70 °F
  • The goldfish’s size and growth rate depend on the size of its environment. The fish can grow between 1 and 6 inches in indoor tanks and up to 14 and even 24 inches in the wild
  • The goldfish can live up to 20 years in captivity in optimal environmental conditions and with a healthy and balanced diet
  • There are a variety of goldfish strains available like fantail, celestial, comet, Ryukin, Oranda, BubbleEye, and many others
  • As hardy as they are, goldfish sometimes experience health problems due to poor tank conditions and sudden changes in water parameters

Today, we will focus on the latter point and discuss the problem of goldfish suddenly dying after a water change. This happens more often with novice aquarists who are unaware of the risks of ensuring the procedure’s safety.

But how will a simple water change hurt your goldfish? Let’s have a look!

Reasons Goldfish is Dying After Water Change

There are pretty much 4 common reasons responsible for sudden death in goldfish when performing a water change:

1. Temperature Shock

If you didn’t know already, goldfish are extremely adaptable when it comes to environmental temperature. They can actually survive extreme water conditions from freezing to tropical temperatures, albeit not for that long.

This is fitting for a species that typically lives in areas with fluctuating parameters and where conditions aren’t always perfect.

The problem arises when the temperature shift is too abrupt, which could cause your goldfish to experience temperature shock.

In severe cases, temperature shock could prove lethal, causing sudden death, no matter how large or healthy the fish is.

The good thing is that there are signs warning of temperature shock, so you have time to react.

These include:

  • Lethargy causes the fish to display static or slow swimming, which mostly happens in extremely cold waters
  • Rapid breathing is noticeable via accelerated gill movements due to hotter-than-usual waters
  • Swimming to the surface more often, since hot waters hold less oxygen, causes your goldfish to suffocate
  • Erratic swimming due to increased water temperature, causing the goldfish to exhaust itself
  • Comatose, causing the goldfish to flip upside down and only display fading vital signs
  • Death

Fortunately, these symptoms give you enough room to act, provided you know what you’re looking for. Ignore them, and your goldfish may experience a rapid and painful death.

2. Drastic Changes in Water Parameters

Water changes are necessary to preserve the stability of any closed aquatic environment. The procedure dilutes ammonia, removes harmful chemicals, and refreshes the environment.

However, performing too frequent or too large water changes can come with drastic consequences. Especially the latter.

For the most part, you shouldn’t change more than 10% of your goldfish’s water weekly, provided you’re not dealing with overcrowding or a too-tight tank.

Changing too much of the aquarium water at once could have the opposite effect, disrupting the environmental balance and affecting your fish as a result.

This happens due to diluting the essential minerals present in the water that form a protective coating and nourish the fish.

Changing too much water at once will also disrupt the tank’s biofilm, destroying the cultures of billions of beneficial bacteria that contribute to the system’s stability.

These bacteria consume ammonia and nitrites, practically sterilizing the system and functioning as scavenging microorganisms.

Displacing too much of the water volume will remove many of these cultures, and you won’t be able to notice the change immediately. But their absence will lead to a fast ammonia buildup, causing ammonia poisoning shortly after and potentially killing your goldfish.

The idea is never to change more than 10, maybe 15% of the goldfish’s water weekly, especially if your fish aren’t overcrowded.

3. Chlorine Poisoning

This problem typically occurs when using tap water. Tap water contains varying amounts of chlorine which is added to sterilize it and make it drinkable to humans.

Chlorine is harmless to us, but it functions as a poison to pretty much all tank fish species. The symptoms can be quite severe, depending on how much chlorine there is in the water.

These may include:

  • Hypoxia, as your fish begins to display symptoms associated with low oxygenation
  • Necrosis of the gills and surrounding tissue
  • Erratic body movements due to neurological issues
  • Sudden death

To prevent chlorine poisoning, I recommend the following options:

  • Avoid tap water altogether – You don’t need to use tap water. There are other safer options out there like RO/DI and distilled water, all of which have been sterilized to remove pathogens, particles, and even harmful chemicals. The problem is that these types of waters are also stripped of all their mineral content, so you need to re-mineralize them in advance.
  • De-chlorinate the water – You can achieve this via several methods, including allowing the water to ‘breathe’ for 48 hours or boiling it to speed up the process. Chlorine will dissipate naturally in contact with the atmosphere. Or you can use a de-chlorinating solution to make sure you eliminate all potential risks.
  • Monitor the chlorine content – This is a must, no matter how careful you may think you are. Even the smallest chlorine content can hurt your goldfish, especially if they’re more sensitive to the chemical or experience problems with their immune system.

As an additional note, water changes aren’t the only cause of chlorine poisoning. Cleaning the filter with tap water can have the same effect, as chlorine will bind to the system and get transported into the tank.

At the same time, chlorine sterilizes the filter, killing off the beneficial biofilm inhabiting the system. This will cause additional issues like ammonia poisoning in the long run.

You should always clean your tank filter with tank water to prevent that.

4. Ammonia Poisoning

This is the direct result of changing too much water at once. If you’ve already cycled your tank and stabilized the system, your next concern should be to preserve its stability.

This means avoiding massive water changes that could disrupt the tank’s ecosystem and kill off the cultures of beneficial bacteria. As I’ve already mentioned, these are responsible for consuming ammonia and nitrites and turning them into nitrates which are less harmful to your goldfish.

Removing them will lead to accelerated ammonia buildup, especially in a goldfish environment, since these are notoriously messy fish.

The signs associated with aggressive ammonia buildup vary depending on how much ammonia we’re talking about.

Your goldfish may experience ammonia stress at first, displaying symptoms like:

  • Gasping for air
  • Looking for hiding spots
  • Becoming more aggressive towards other tank mates
  • Showcasing an inflamed anus and bulgy eyes
  • Noticeable lethargy and lack of interest in common activities
  • Reduced or loss of appetite

If the situation is more severe, the fish may experience ammonia poisoning, at which point the symptoms will become more drastic.

These include:

  • Red or bloody gills
  • Refusal to eat
  • Bloody patches or sores all over the body
  • Coma-like lethargy with minimal movement
  • Erratic swimming
  • Laying on the bottom of the tank with clamped fins
  • Internal and external hemorrhage
  • Death

To prevent this issue, stick to the recommended water change amount. Don’t exceed 10% in the water volume, and never clean your tank, substrate, rocks, or filter with chemicals that could kill the bacteria inhabiting those areas.

How to Save Dying Goldfish After Water Change?

What you should do depends on why your goldfish are dying. So, you should first assess the situation to figure out the cause.

Here are some basic recommendations, depending on the issue:

Ammonia toxicity/poisoning

  • 25-30% water change to dilute the ammonia
  • Use chemical filtration components like activated charcoal
  • Use antibiotics in case of ammonia-related infections
  • Improve water oxygenation via air stones
  • Consider removing the goldfish from the environment to prevent further damages

Chlorine poisoning

Remove the fish from the tank – Your fish will recover naturally in a stable and healthy system.

Unfortunately, chlorine-related tissue damages are irreversible and prolonged exposure will kill your fish. If the damages aren’t too extensive, your goldfish will recover.

Temperature shock

Measure the tank’s temperature and get the heater going – The goldfish should recover fast if the temperature shock isn’t too aggressive, in which case the fish may experience sudden death.

Dangerous shifts in water parameters

  • Monitor and adjust water parameters as necessary
  • If the water parameters are too far away from the fish’s comfort zone, relocate the fish into another tank temporarily

If I were to provide my personal opinion, that would be to always have a backup plan in case the water change doesn’t go as intended.

This implies having a secondary cycled tank that you can use in case of emergency. This investment may save your goldfish’s lives when the time comes.

Are Goldfish Sensitive to Water Changes?

Not particularly, but it depends on how you perform the water changes. Ideally, you should only change around 10 to 15% of the water weekly.

This is enough to freshen up the goldfish’s environment but not enough to cause significant disruptions in water parameters.

You may even need to up those values to 20% if you have multiple goldfish producing a lot of waste so that the filter cannot handle to load.

Other than that, the goldfish will handle water changes quite easily, provided everything goes as planned. If not, at least now you know what you can do.


Goldfish are generally hardy fish that can handle a variety of water parameters, but you shouldn’t test their limits.

They don’t cope too well with abrupt shifts in their environmental conditions, and improper water changes create the perfect conditions for that.

To conclude:

  • Always use chlorine-free water enriched with essential minerals to nourish your fish
  • Verify the water’s temperature to prevent temperature stress or shock
  • Stick to 10-15% water changes to prevent dangerous ammonia buildup
  • Don’t perform more than 1 water change per week for the same reasons unless absolutely necessary
  • Double-check the water parameters before and after performing the water change
  • Monitor your goldfish for a couple of days following the water change to make sure everything is fine

Other than that, you should always act in case your goldfish displays abnormal behaviors like lethargy, lack of appetite, difficulty breathing, or any other worrying symptom.

Nothing beats early treatment. Except for prevention, of course.

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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