Why are my fish dying? 11 Causes and How to Avoid Them

So you just purchased your first goldfish...

You bring it home, only to find it floating belly up a few days later.

Perhaps this is not your first fish funeral. You keep finding your dead fish - it's as if you've turned into a fish serial murderer.

You might start shouting at this point...

Why are my fish dying?

There are numerous reasons why the fish in your tank continue to die. Stress, improper tank setup, overfeeding, illnesses, and other factors are all possible causes.

To a novice, it may appear that your fish perished for no apparent reason.

So, today, I'll solve the enigma of why your pet fish keeps dying.

Stress is the number one killer of pet fish!

It's not good for humans, and it's certainly not good for fish.

Fish eventually die as a result of stress. It's simply a matter of time. It could take days or weeks, but one thing is certain: your fish's days are numbered.

Unfortunately, stress makes it difficult for a novice to figure out why an otherwise healthy fish perished.

Imagine coming home to find that every fish in your aquarium has died.

You'd probably reflect on what you accomplished that day. Your fish was fine yesterday, so the cause of death had to have occurred today, right?

The reason for your fish's death may have occurred weeks earlier, and your fish may have done their hardest to survive the entire time before finally succumbing to the struggle.

Stress, you see, is not an instant killer. It's more like a ticking time bomb.

Because fish do not show emotion or expression, a newbie may not realize their fish is on the verge of death until it dies.

So, let's look at some of the most prevalent stressors that can cause your pet fish to die prematurely.

1. Your aquarium was not correctly set up

It's simple to set up an aquarium: get a glass tank, fill it with water, then add your de-chlorinator. Isn't it only a matter of adding your fish now?

No, it doesn't.

You might not be able to add fish to your new aquarium for a few weeks, believe it or not.

You can, but there is a good chance they will die soon.

The very first thing you should do is cycle your aquarium.

You see, your aquarium is home to more than just fish. Bacteria that break down and eliminate waste from your aquarium are also present in a tank with excellent water quality.

Simply put, this bacterium keeps your fish safe in the water.

There is only one problem: this bacteria does not exist in a brand-new tank. If you add your fish now, they may grow stressed and perish since they require a stress-free environment.

So, you'll want to put these bacteria into your tank water as soon as possible, ideally before you acquire your fish.

Cycling your aquarium is the process of establishing this healthy bacteria.

Cycling your aquarium should ideally be done before purchasing your fish, as the procedure can take several weeks. The Fishless Cycling Guide from FishLab will lead you through the processes to make standard aquarium water safe for your fish.

If it is too late and you have already purchased fish, you can try a fish-in cycle. To offer your fish the best chance of survival, consult our Fish-in Cycle Guide.

Fish-in cycling is extremely stressful for fish, and your fish will most likely die as a result. I strongly advise cycling your new tank before purchasing fish

2. Your tank layout is inappropriate for your fish

Your tank should be set properly to accommodate the fish you keep.

In a saltwater tank, a freshwater fish will perish. A fish that enjoys warm water will likely perish in cold water.

However, just because your fish isn't dead yet doesn't indicate your setup is correct. Even if your fish looks to be surviving, it may be overly agitated. And, as previously said, stress can result in premature mortality.

Consider the betta fish. They enjoy quiet water, therefore vigorous water movements, such as that produced by a filter nozzle or bubbler, can stress them out.

A pleco, on the other hand, needs a lot of hiding spots. Plecos might grow nervous if they don't have a safe place to hide.

Meanwhile, African cichlids require hard alkaline water, whereas cardinal tetras flourish in soft, acidic water.

Failure to offer a proper atmosphere and perfect temperature for your fish can result in it dying far too soon.

Because no two fish are alike, research your fish's preferences before introducing them to your aquarium. Where one fish thrives, another becomes anxious and eventually dies.

3. Your aquarium is insufficiently sized

Do you have too many fish in your aquarium? For a variety of reasons, an overcrowded tank is practically a death sentence for your fish, and perhaps all you need is a larger tank.

First, remember the good bacteria I mentioned in the previous part, the one that breaks down waste to keep the water clean?

It can only break down so much at once. In general, the larger the tank (and filter), the more beneficial bacteria will be present to break down waste.

As a result, the more fish you add to your aquarium, the more poop there will be. If your fish are pooping faster than the helpful bacteria can break it down, your water will quickly become toxic, killing all of your fish.

For beginners, this most usually occurs when betta or goldfish are maintained in an aquarium that is too tiny, although it can occur in any size aquarium.

But it's not just overpopulation that's a problem. A small tank with little room to roam can cause a fish to get anxious and die suddenly.

Finally, fish tank owners should be aware that overpopulation might cause their fish to choke...

Fish, like you and me, require oxygen to breathe; without it, they will perish.

The issue is that your aquarium only has so much oxygen. And if you have too many fish, they will suffocate since they will be breathing quicker than the oxygen can be replaced.

Remember that fish grow. Just because your fish is little currently does not guarantee it will remain that indefinitely. Your goldfish may be less than 2 inches long now, but it can grow to reach 10 inches long!

4. You're combining fish that aren't compatible

Not every fish gets along.

While you may have had the best of intentions, you may have unintentionally produced an underwater cage battle rather than a calm home for your fish.

Fish like cichlids, Chinese algae eaters, blue gourami, and male betta, which are widely owned by novices, can be territorial, quarrel over food, and even attack or bully other fish.

Your fish would be able to quickly escape its bully in the wild. Unfortunately, your aquarium is substantially smaller, so your fish has no alternative but to flee for its life at all hours of the day and night.

Being chased and nipped at by a territorial fish is stressful and will most likely cause your fish to die sooner than it should.

To avoid aquarium fish fatalities, ask about the compatibility of any new fish you purchase for your aquarium.

5. You overfeed your fish

Overfeeding is another exceedingly common cause of death.

A newbie is often surprised by how little food fish eat.

Consider a single betta - you know those micro-pellets they eat? Your betta only requires three to stay full. You are overfeeding if you eat more than this.

Overfeeding, on the other hand, is terrible for your fish since it causes the water to become toxic, which can lead to water poisoning.

Simply put, whatever goes into your fish must come out. The more food you give your fish, the more feces they produce. Furthermore, uneaten food decays, producing extra waste that degrades water quality.

This trash quickly pollutes your water and becomes toxic, killing your fish.

You should also ensure that you are feeding your fish the proper food. A meat eater will not live long if he eats fish food produced from veggies.

6. You do not do routine tank maintenance

You should clean your aquarium at least once every week or two. This usually entails wiping down the glass, cleaning the substrate, and double-checking that everything is in functioning order.

The most crucial maintenance, though, is a water change.

What about you and me? We enjoy the luxury of being able to keep our home clean by throwing out rubbish and flushing the toilet.

Your fish, on the other hand, will have to swim in it. If garbage continues to accumulate and you do nothing to address it, water quality issues will arise, putting your fish at risk of dying.

As a result, you should replace the water in your aquarium once a week.

You can maintain your fish happy and healthy by removing 10-20% of the gunked-up aquarium water in your tank each week and replacing it with new water.

7. Your aquarium had frequent water changes

Rapid changes in water temperature, PH, or salinity can shock a fish, leading to death.

As a result, you should acclimatize any fish you bring home before putting them in your tank. The temperature differential between the water in the bag and the water in your tank can be significant.

Similarly, if you are making a significant water change in your aquarium, the temperature of the water added should not be too different from the temperature of the aquarium.

Any alterations or shifts in water parameters made to your aquarium should be gradual and consistent.

8. killed your aquarium's beneficial microbes

As I mentioned previously, after cycling your tank, you will have beneficial bacteria in your aquarium. Furthermore, this bacterium breaks down waste, which would otherwise make your fish sick and eventually kill them.

This beneficial bacteria is mostly found in your filter and substrate.

If these bacteria die, the water in your aquarium will quickly become toxic and kill your fish.

What causes this microorganism to die? It is frequently done by accident.

If you neglect to add a de-chlorinator during a water change, you will kill the entire bacteria colony because chlorine is a bacterium killer (and a fish killer as well!).

Similarly, rinsing your filter with regular or chlorinated water will have the same effect.

Another method is to replace your tank filter or filter material. Because your new filter has no beneficial bacteria, waste will quickly accumulate in fatal amounts.

9. Your fish had a difficult journey

Can you imagine strolling outside and suddenly being kidnapped and imprisoned in a little cell?

Doesn't that seem stressful?

It sounds like a scenario from a horror film! However, it is not dissimilar to the voyage that many fish destined for aquariums take.

If your fish was caught in the wild, it traveled halfway around the world just to wind up in a little tank in your local fish store, which is a significant difference.

Probably, the fish you purchase are already anxious as a result of their long travel.

Similarly, the final journey from the fish market to your house might be lethal.

That plastic bag in which you transport your fish? It can only hold so much oxygen.

Furthermore, excessive shaking of the bag will undoubtedly stress your fish, resulting in an unhealthy fish.

10. Your fish is infected or has a parasite

According to research, people are more prone to develop a cold when they are stressed.

According to studies, the same is true for fish.

A healthy, stress-free fish has a powerful immune system, which means that its skin, scales, and slime coat are more than capable of fighting off any sickness or parasite.

A stressed fish, on the other hand, is more susceptible to illness and disease, and a sick fish eventually dies.

If you feel your fish has a disease, you can learn more about freshwater ich and columnar in our articles. You can also learn about planaria, a pest.

Keeping your fish stress-free goes a long way toward ensuring their survival.

11. Your fish is becoming old

Finally, there is old age.

To be honest, this is perhaps the least likely solution for a new aquarium owner asking why their fish perished.

However, if your fish had a previous owner, it is possible that your fish died as a result of living a full life.

After all, no fish can live forever, and your energetic little fish will eventually pass away.

Yes, regardless of how well you care for your fish... There is no getting away from old age.

The length of time it takes for your fish to die is entirely dependent on the breed. Bettas can live for four years, whereas healthy goldfish can live for ten.


To be honest, in my 30 years of fishkeeping, the majority of fish deaths may be attributed to the owner making a mistake or simply not knowing any better.

Accidents do happen. And there is nothing to be embarrassed about.

Don't beat yourself up if you committed a mistake that caused your fish to die. And don't use it as an excuse not to breed another fish!

Instead, view it as a learning opportunity. You may now give your future fish a long and happy life!

Can you think of any other possible causes for a fish's death? Please let me know in the comments!

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