Why Are My Goldfish Dying (And How To Stop Them Dying)?

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There are essentially three main reasons your goldfish might be dying unexpectedly. 1, low oxygen levels in the water, 2, disease, and 3, poor water quality. A fourth reason your goldfish might be dying is low, or high water temperature

It can be so disheartening when your goldfish start to die for no apparent reason. You sit there looking into your goldfish aquarium thinking you haven’t changed anything.

Main Reasons Goldfish Die Suddenly

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

Are You Over Feeding Your Goldfish?

Goldfish are greedy feeders. They will eat constantly. You must have seen them when you lift the lid to your aquarium, they are straight there saying ‘feed us, feed us!’. Worse than that, they will eat just about anything you put into their aquarium.

The problem, if you overfeed your goldfish, is any food that the goldfish don’t eat will sink down to the substrate, or behind the plants, or maybe into crevices between rocks or decorations. This uneaten food will then just rot down and foul your aquarium water.

Another situation where overfeeding can arise is when you are on vacation. If you have left a family member or friend to feed your goldfish, I can almost guarantee they will have overfed your goldfish. Non-fishkeepers always put too much food in. It doesn’t matter what instructions you leave!

To prevent issues with overfeeding, only ever give your goldfish as much food as they will eat within a couple of minutes. If they eat it all quickly, add a little bit more. Repeat until your goldfish lose interest in the food.

If you believe your goldfish are dying due to overfeeding, start by holding back food for a couple of days. Don’t worry, your goldfish will be fine. Next, use a test kit to test the quality of your water.

If you have high ammonia, nitrite, or nitrate, change 25% to 50% of your aquarium water, replacing it with fresh tap water (dechlorinated if necessary). In the future, feed your goldfish less, following the suggested feeding method above.

Test Your Goldfish Aquarium Water Temperature

Water Temperature? But goldfish are cold-water fish, right? Well, that is partially true. Common goldfish and comets like their water cool. For most areas, room temperature is fine. Fancy goldfish like Ranchus, Fantails and Lionheads actually like their water a little warmer. Fancy goldfish need aquarium water to be around 70°F to 76°F. Keeping the water at this temperature aids their digestion.

If your water is too cold, or too hot, your goldfish may start to suffer stress. This stress can allow pests or diseases to take advantage of your goldfish. A stressed, diseased goldfish can die very quickly.

To prevent goldfish from dying due to water temperature issues, fit your goldfish aquarium with a good quality digital thermometer.

Every time you feed your goldfish, or just interact with your aquarium in any way, check the temperature on the digital thermometer. If the temperature is too high, check your aquarium heater (if using one).

Has it stuck in the ‘on’ position meaning it is slowly cooking your goldfish? If you’re not using a heater, but the temperature is still too high, is the aquarium too close to a heater? If the water temperature is high purely due to the weather, try pointing a desk fan blowing across the water surface. An effect known as ‘evaporative cooling’ will help bring your aquarium temperature right down.

If your water temperature is too low, try adding an aquarium heater. Set the heater to a relatively low setting. Your goldfish will certainly thank you for the slight increase in water temperature.

Oxygen Levels

It may surprise people to learn, goldfish need oxygen to survive, just like we do. The only difference is goldfish have to draw their oxygen from the water rather than the air. If there isn’t enough oxygen in your aquarium, your goldfish will possibly be gasping at the surface.

The amount of oxygen in the water can vary based on a number of different factors. Water temperature can affect oxygen levels. Warm water holds less oxygen than cold water. If your goldfish tank is too warm, there may not be enough oxygen available.

Another factor affecting oxygen levels is water quality. Poor quality water also holds less oxygen. Testing your aquarium water with a simple test kit will let you know if your water quality has dropped.

If you find your goldfish gasping at the surface, immediately change 25% to 50% of the aquarium water by draining the aquarium down and replacing the existing water with fresh tap water, (dechlorinated if required).

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

Poor Water Quality

Poor water quality is possibly the number one killer of goldfish today! Goldfish are heavy feeders. They eat a lot, which means they also poop a lot! Add the fact they generate a lot of waste to the fact that we tend to keep far too many goldfish in a tank, and you have a recipe for poor water quality.

A goldfish aquarium requires a surprising amount of filtration to keep the water clean and clear. When there isn’t enough filtration to cope with all the waste from the goldfish, the water quality will deteriorate.

Fish waste is very high in ammonia. Ammonia is extremely toxic to fish. Luckily, the bacteria that live in our aquarium filters break down that ammonia and turn it into nitrite, which is less toxic to fish. Another type of bacteria then converts that nitrite to nitrate, which is less toxic still.

Once the filter has converted the fish waste all the way to nitrate, it is our job as fish keepers to remove that nitrate via water changes before it becomes harmful to our goldfish. It is recommended that we change 25% to 50% of the water in our goldfish aquariums every week.

This water change procedure is probably the number one thing you can do to keep your goldfish healthy.

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

Aquarium Nitrogen Cycle (source: Wikipedia)

Aquarium Cycling Issues

Aquariums must be ‘cycled’ before we add fish. Cycling an aquarium is a hot topic in fishkeeping at the moment. The topic itself is too big to cover in a small article like this one, so instead, take a moment to watch the video below from KG Tropicals.


Diseases are another major reason goldfish die. Goldfish are bred in massive quantities. They are shipped in bulk and then kept in poor conditions at both wholesalers and stores. All these factors can mean your new goldfish already has either pests or disease in its body when you bought it.

If your goldfish is new to you, seriously consider trying some precautionary treats like API General Cure (which is really good value on Amazon at the moment).

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 goldfish aquarium. Ammonia is very toxic to goldfish.

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

  • Overfeeding: Personally, feeding my goldfish is the most enjoyable part of the hobby. However, I know from first hand experience that it is very easy to overfeed your goldfish. They come to the surface, begging for food. The problem with overfeeding is that any uneaten food sinks to the bottom of the aquarium and gets stuck behind decorations or falls into the crevices between rocks. This uneaten food then rots down, releasing ammonia into your aquarium. If enough uneaten food sits, rotting in the aquarium, the ammonia will spike. This spike can be fatal to your goldfish. To prevent overfeeding, only add as much food to your goldfish aquarium as they will eat within a couple of minutes.

  • Adding New Fish: If you add new fish to your aquarium, you instantly increase the bioload, which means more ammonia is released into your aquarium through the fish waste. To prevent an ammonia spike when you add new fish, don’t add too many at once. Adding a couple of goldfish at a time will allow the bacteria in your filter to increase in number to handle the extra fish waste

  • Dead Fish: If one of your goldfish dies, but you don’t notice, it will start to break down in the aquarium. As it breaks down it releases ammonia into the water. This ammonia is toxic to the other fish, which can lead to other fish dying, as these fish die they also begin to release ammonia, resulting in a domino effect, killing all your fish.

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.


Aquarium Overstocking

It is very easy to overstock a goldfish aquarium. Goldfish require more space than people think. A 30-gallon aquarium is a minimum for a single goldfish, and many people would argue a 30 gallon isn’t large enough.

Typically what we do is keep 3 or 4 goldfish in a 30 gallon or smaller. We then become slaves to the water changes to try and keep the nitrate levels in the aquarium low enough. Goldfish are very messy eaters. They eat a lot and they poop a lot.

If your goldfish aquarium is overstocked, your goldfish may all die in a short space of time. Unless you are on top of your water changes, test the aquarium water on a regular basis to check nitrate levels aren’t elevated.

If you believe your goldfish are dying due to overstocking, firstly carry out some large water changes to bring down the nitrate levels, then look to either purchase a larger aquarium or rehome some of the goldfish.

Poor Genetics

Goldfish are bred in their millions every year. Often there is no attention paid to the genetics of the goldfish. ‘Pile them high and sell them cheap’ seems to be the attitude many commercial breeders and stores take.

To try to avoid buying a goldfish with poor genetics, take your time in the store. Look for goldfish which seem active and swimming without issues. Avoid any that have missing or damaged fins.

If the body is an odd shape, don’t buy it. It is impossible to know the quality of the parentage of an individual goldfish, so just try to buy fish that look healthy and appear active.

Never be tempted to offer a home to the sick-looking, bent one hiding in the corner.


Stress can be a major killer of goldfish. Goldfish can become stressed when water quality is poor or if their aquarium is overcrowded. External factors like children banging on the aquarium glass constantly, or the tank is situated next to loud stereo speakers or television can cause your goldfish stress.

Another major cause of stress is male goldfish chasing female goldfish to try and spawn. Goldfish are very persistent spawners. The males will chase the females around for hours. If your tank has 3 males to every female she will never get a break.

Try to keep the ratios 1 male to 3 females, that way his advances are shared equally amongst 3 females rather than a single fish. For more information on sexing goldfish, have a look at this article.

How To Keep Your Goldfish Healthy

Now we have looked at some of the reasons goldfish die unexpectedly, let us look at some of the ways we can help keep them healthy;

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

Regular Water Changes

As discussed above, goldfish are big fish, they eat a lot and they poop a lot. All of these factors add up to a potential problem with high nitrate levels. When nitrate levels build up to more than 100ppm they can start to have a detrimental effect on your goldfish.

The only realistic way we as home hobbyists can hope to lower nitrate levels is through regular water changes.

To keep things simple, try to get into a routine whereby you change 25% to 50% of the water every 2 to 3 weeks. For your goldfish setup, changing water that frequently may be too often or not enough.

The only real way to know is to test your water on a regular basis, monitor the nitrate levels, then change water as and when the tests show the level of nitrates is too high. I have used the API Master Test Kit for many years.

It is reliable and simple to use. If you need to test multiple aquariums, test strips can be a better way to go.

When water changing, be sure to use a suitable dechlorinator before adding the water back into the aquarium.

Preventing And Treating Diseases Quickly

Preventing diseases from entering your goldfish aquarium in the first place is your number one defense against pests and diseases killing your goldfish. There are a couple of things we can do.

  • Quarantine New Fish: Quarantining new fish is probably the number one way to keep pests and diseases out of your goldfish aquarium. To quarantine new fish, set up a dedicated quarantine tank. A quarantine tank doesn’t have to be anything fancy. A plastic box that holds water is sufficient. It will need a heater and a filter, but that is all. The longer you quarantine your new fish the better, but aim for a minimum of 1 week. If you are anything like me, a week is forever when you have new fish to add to your tank. When you have fish in quarantine, consider some preventative treatments like deworming.

  • Buying From a Reputable Vendor: Where you buy your goldfish from can have a major influence on the quality of the fish. Find a local fish store and spend some time getting to know the store and the staff. If they themselves buy quality stock, and then put them through quarantine before you buy them, and quarantine them, there is a much lower chance your new goldfish will bring pests or diseases into your aquarium.

  • Identify and Treating Diseases Quickly: By regularly observing your goldfish, and reacting quickly when you notice something isn’t right, you can hopefully start treating any diseases before they really take a foothold in your goldfish tank. If you see signs of a disease or pests in your aquarium. Make a note of the symptoms and consult your local fish store. They will hopefully be able to recommend suitable medication or treatments.

Feeding A Balanced Diet

Goldfish are omnivores. This means they need a diet based on both meat and plant matter. Feeding your goldfish a good quality flake or pellet food is a great base to a balanced diet. As well as feeding dry food, consider offering either live or frozen food on a regular basis too.

Goldfish will relish bloodworms, daphnia, and brine shrimp. They will also eat Repashy Super Gold gel food. Repashy has high-quality ingredients and keeps in the fridge for several days or in the freezer for a few months.

Properly Maintaining The Aquarium

Keeping up with your aquarium maintenance can seem like a chore, but it can make a real difference to the health of your goldfish. Regularly cleaning your filter can keep it running efficiently and processing your goldfish’s waste effectively.

Checking filter intakes aren’t blocked by leaves can have a similar effect. Gravel vacuuming the substrate physically removes a surprising amount of detritus from your aquarium, which will help stop nitrates from building up too quickly or too high.

In Conclusion

Goldfish are generally hardy fish. They don’t usually die for no reason. If you have lost one or more goldfish, work quickly to try to establish why. Assuming you don’t see any obvious signs of disease, test your aquarium water.

Check for an ammonia spike or excessively high nitrates. Finding the route cause will hopefully stop more fish from dying unnecessarily.

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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