Why Does My Fish Keep Dying? Top 10 Causes Of Fish Death

Why Does My Fish Keep Dying? Does All Might Die…

With their brilliant colors and soothing swimming motions, fish can be delightful pets, but it can be unnerving and distressing when fish die for no apparent reason.

There is always a reason. Understanding the most common causes of aquarium fish deaths can help fish owners to prevent mortal mishaps in their tanks.

Why Do My Fish Die?

Main Causes of Aquarium Fish Mortality

Usually fish are healthy and it is very rare for an aquarium to be subject to mass deaths without any other reasons at work.

Fish are very delicate animals that need a lot of tender loving care. They are extremely delicate beyond your imagination, even the ‘low maintenance’ fish like bettas. Platies have primary tank requirements that when not properly met then it would make them very sick, and might even cause them to die.

Every hobbyist fish keepers experience unexpected and sudden fish death, regardless of their fish tank upkeep expertise and experience.

There is always a reason for the fish to become ill. It only takes a little investigation to find out the actual reason.

If there is any fish death in your aquarium, immediately you should try to find out the reason why it happened before there is more loss of fish in the tank. In a major portion of the time death of fish is not caused by infection or disease, but by changes to the aquarium environment.

All of the common reasons are going to be explored why fish might die and detect the ways to stop it from happening further.

Why Do My Fish Die Causes Of Fish Death

There are different causes that could create tank die-offs. Such as:

1. Aquarium Water Temperature

A range of temperatures in your fish tank can be tolerated by most aquarium fish, but dramatic or sudden temperature changes may cause them stress. It can make your fish much more susceptible to illness. If there are any drastic changes then it can be quickly fatal.

Regarding the range of aquarium water temperatures, your fish are very sensitive. However, this temperature does not compulsorily require it to be exact. Every fish has its own water temperature range in which they thrive best. Makes sense right?

In the wild environment, there is no other way of keeping the rivers, streams, oceans, or lakes at a fixed temperature. Therefore it is logical that fish can be flexible to some extent in a captive environment.

One essential thing to observe is rapid fluctuations in aquarium water temperature. Here we can see some problems and deterioration in your fish health that are kept in an aquarium.

Although the temperatures of your tank water might be changing within a ‘safe’ range, the fluctuations may be happening too fast for the fish to adapt and manage to cause them distress.

To Prevent: Check the tank heater for proper function daily, and position the tank away from anything that might affect its temperatures, such as heating or cooling vents, sunny windows, or drafty areas.

To stop some outside factors – such as lighting – from changing the aquarium temperature, introduce your fish tank in a quiet, dark corner of the room, far from direct sunlight.

This can prevent any outside weather fluctuations from happening and thus keep the aquarium’s temperature fixed or stable.

On the other hand, the problem could be that you will require some species of fish in a shared aquarium that is not temperature compatible. It also happens that multiple species may always be swimming in the water which is not at the perfect temperature.

How to avoid this? Only introduce together compatible fish in the aquarium-like ones which will be available in the wild environment in the same place as one another.

2. Quality of Tank Water

The total water quality in an aquarium is a key factor that affects the health of a fish. The salinity of the water (for saltwater aquariums), pH levels, filtration efficiency, and other quality issues must be maintained within ideal levels, or any one of them could be responsible for dead fish.

How To Prevent: Research and find the proper water quality for the fish, and take action to balance decent quality levels to protect fish’s health.

While talking about the quality of water in your tank, we are referring to stuff like the pH levels in the tank water, the water filtration process, salinity, and the use of the wrong water type in a fish tank.

Your fish could suffer tremendously if just one of the above factors is a little off.

The filter of your fish tank must contain a build-up of beneficial bacteria which are essential for maintaining a maximum standard of water quality.

My suggestion is to install a new filter by removing the old filter in the tank, regardless of whether the aquarium is already cycled or not is a big issue.

To avoid filter issues in your aquarium, have the old filter and new filter running together in the tank for at least a week to help the new filter colonize.

To support the process a little bit, you can add water conditioners and beneficial tank bacteria to the aquarium water. If you are fully confident that the new filter is ready, then remove the old one from your tank.

Levels of water pH are equally important as having a colonized fish tank filter. The high amount of pH shows that the tank water is alkaline. On the other hand, lower pH levels indicate acidity in the water. Your fish and aquarium plants can have their preferred pH ranges, also not honoring that could result in your fish’s death.

There are so many things that could change the level of pH in your aquarium water, it includes adding new fish to your aquarium. The highly common causes your pH levels may have been altered is due to adding some chemicals to the water of the tank, changing its substrate, or your aquarium decorations.

Some particular substrates like limestone and coral may dissolve into the aquarium water and alter the pH levels dramatically.

If you do not have researched your aquarium decorations and substrate thoroughly before buying it, you might run into this big problem without any intention.

API FRESHWATER MASTER TEST KIT 800-Test Freshwater Aquarium Water Master Test Kit, White, Single, Multi-colored
  • Contains one (1) API FRESHWATER MASTER TEST KIT 800-Test Freshwater...
  • Helps monitor water quality and prevent invisible water problems that...
  • Accurately monitors 5 most vital water parameters levels in freshwater...
  • Designed for use in freshwater aquariums only
  • Use for weekly monitoring and when water or fish problems appear

Last update on 2024-07-11 / Affiliate links / Images from Amazon Product Advertising API

3. New Tank Syndrome

Heavy concentrations of nitrates and ammonium in the water can be fatal before a tank has developed the appropriate chemistry to support healthy fish.

Sometimes, natural bacteria in the water will balance out these contaminants. However, until that balance is achieved, fish may die unexpectedly.

How To Prevent: Check a new tank regularly and run a test for ammonium and nitrate levels, and alter the water as required to decrease the levels so that they are safe and healthy for your fish.

In reality, fish don’t like crystal clear and fully sterile water. Think it like this: can fish swim in the pristine, super-clean water in the wild environment? No, it can only make sense that their aquarium shouldn’t be fully sterile either.

I am not saying you have to let your aquarium get filthy, but beneficial bacteria are most important to your fish tank, as is maintaining the perfect nitrogen levels in the fish tank.

The unnecessary fish produced is broken down by the beneficial bacteria living in your aquarium. If there are not a sufficient amount of bacteria, or no bacteria in your aquarium, the ammonia levels in the tank water spike. This is toxic to the fish in your tank. If you don’t restore the balance in the aquarium, this may cause them to be very sick.

This system of recycling is called the nitrogen cycle. The slang reference for this recycling process amongst aquarium enthusiasts is to ‘cycle’ the tank.

Aquariums must have to be cycled after each partial or complete water change when new fish are placed in your aquarium. If you fail to do this then it will result in your fish getting new tank syndrome.

4. Rapid Water Changes are Another Reason

In an established and healthy aquarium, water chemistry is very carefully balanced with plants, resident fish, and bacteria.

Suddenly changing bulk quantities of water can disrupt that chemistry and may shock your fish, causing death. Occasionally tank water does require to be altered, but sudden, large changes would be harmful.

How to Save Dying Fish After Water Change? Change your aquarium water slowly, swapping only small quantities at a single time and waiting 2 to 3 days before altering or changing additional water so the fish could acclimate to gradual chemistry changes.

5. Overfeeding Your Fish

You do not have to provide your fish with three meals a day. Overfeeding your fish not only wastes food, but spoiled food can also easily contaminate your tank. It can unbalance its delicate chemistry.

A few mouthfuls of food are required by fish every day, and even though your fish might beg, they don’t need substantial treats.

How To Prevent: Plan a hard feeding routine and you should not deviate from that strict schedule. You have to adjust the feeding quantities until the fish consume all the food within 1-2 minutes. Do not give them extra food.

Fish don’t have any idea when they are full. They could only eat repeatedly until they pass out. Serious health issues are caused by overfeeding your fish.

The additional waste that is caused by the fish consuming more than they should change the chemistry of the tank water, as does unconsumed decaying food which sinks to the bottom of the aquarium.

Here are some signs to see which shows your fish have been overfed:

  1. Filter systems can get clogged with old food, interfering with the natural balance of the aquarium.
  2. If you start to notice small white or brown worms in the water, you need to respond and treat your tank right away.

Flatworms are not so harmful to the fish in any way, however, the flatworms are an indication that you are giving your fish way too much necessary food.

If you observe flatworms in a breeding aquarium or around the fish eggs, remove them as soon as possible, otherwise, they may eat your fish eggs.

  1. Cloudy aquarium water is another sign of overfed fish. Unless it is absolutely important, don’t be so quick to perform tank water changes.

A simpler way would be to stop feeding the fish for 2 or 3 days. This will help to clear up the water. (Most of the fish will be fine with no food for a few days.)

  1. Food can get stuck in decorations and in the substrate, especially if you have a gravel substrate. Algae and mold can eventually grow if the issue is left unsolved.
  2. Fish with sensitive stomachs could catch diseases from overfeeding. Take the cichlid species, for example, they can get fatty liver disease from being overfed.

Feeding the fish an inappropriate diet could be another food issue. All fish do not like to eat dry fish food flakes. Some large-sized fish feed on super worms and others prefer water fleas or bloodworms.

The actual thing is that plenty of fish has varied and peculiar diets to follow. Find out the best diet for your fish and stick to that food to make sure they are healthy.

6. Dirty Fish Tank

Your dirty fish tank changes the levels of nitrites, ammonia, and also nitrates in your aquarium. It is particularly necessary to keep all these compounds fixed with a view to maintaining a balanced marine environment in your fish tank.

A few fish are more resistant than others. Again some fish can show signs of being distressed or sick when the aquarium is dirty, while weaker fish will die within a very short period of time with no prior warnings or signs.

To get rid of this problem, the best solution is to write up a tank cleaning schedule. That’s how you can not forget when you are required to create water changes or when you have to clean the accessories and substrate.

7. Fish Injury

There are some fish that have very delicate fins, for example, the betta fish. The easy-going temperament and striking long fins make betta fish extremely popular.

Many fish find the fins of betta so attractive that they nibble on the fins and making them injured and making them a big deal of distress.

8. The Wrong Aquarium Substrate

The aquarium substrate you choose is more necessary than you imagine. There are some bottom dwellers that have extremely sensitive underbellies and may get damaged if put in an aquarium with a sharp gravel substrate.

Sand isn’t appropriate for all fish species either. Sand can clog up the passageways of fish and can get stuck inside of them. This is almost irreparable if it happens to your fish.

9. More Unexpected Toxins

Even a very small quantity of toxic contaminants might be fatal to your fish, such as cleaning chemicals, perfume, bug spray, hand lotion, soap, and a few other seemingly innocuous materials. If the aquarium water becomes contaminated with these types of items, your fish will heavily suffer.

How To Prevent: By using a good cover protect your aquarium from unintentional contamination and keep toxins away from the fish tank.

Always remember to wash your hands with non-antibacterial, unscented, soap before reaching into the aquarium water for any important reason.

10. Unhealthy Fish in Aquarium

If any fish is sick, stressed out, or in poor condition before being introduced to a fish tank, even the best aquarium environment may not keep them alive.

How To Prevent: Learn about the symptoms of usual diseases for fish you want to purchase. Always purchase fish from reputable, experienced, breeders, or pet dealers.

Select only the healthiest fish in the best condition so they will better be able to withstand the stresses of transportation and incorporate them into a new tank.

Keeping fish is a great experience that can’t be expressed. Again knowing exactly the reason why fish might die keeps that experience great by avoiding the most usual causes of aquarium fish deaths.


Why do my fish keep dying?

There are a lot of reasons the fish in your tank are dying. Some of them are stress, inadequate tank layout, excessive feeding and diseases, and a lot more. For a novice, it may appear as if your fish died without reason at all.

What do I do if all my fish are dying?

1) Check water to determine underlying quality issues and deal with them accordingly
2) Examine for signs of deoxygenation or fish breathing quickly.
3) Check for obvious physical signs on dead fish or on the entire tank to help you determine the cause.
4) Take into consideration how old the fish is, if only one of them has died, but the others seem healthy

Why are my fish dying but the water is good?

The fish are dying, even when the water passes tests flawlessly due to common causes like the wrong side of the aquarium, incompatible with mates, temperature fluctuations, etc. The fish could reach their old age.

Should I change water after fish died?

The most effective way to clean your aquarium of the toxins accumulated from dead fish is to exchange some of the water. It isn’t necessary to clean all of the tanks however getting rid of any contaminants that may have leaked to the surface of your water are vital.

How do I increase oxygen in my fish tank?

Ways To Increase Oxygen In A Fish Tank
1) Pouring Water From A Height. The easiest way to increase oxygen levels and to aerate the water in a fish aquarium is slowly pouring water down from a certain height
2) Large Water Changes
3) Stirring The Water Manually
4) Adding Ice Cubes & Using A Fan
5) Using A Battery-powered Air Pump

Can dying fish be revived?

A dead fish can’t be resurrected. However, nothing is wasted if there are un-lifeless fillets in your freezer from the summer of last year. They are becoming less appealing to the table with each passing day.


Unfortunately, losing fish is a part of the sport. It’s pretty easy to determine if an animal has died or not however it isn’t always easy to know if they’re beyond the point of recovery.

The most obvious signs of a dying and sick fish include difficulty breathing, irregular movement, a pale color of the body, or a loss of appetite. These signs could be a result of improper parameters for water, temperature, and stress or injuries. Remember that fish die due to age-related diseases, too.

There are a variety of remedies available to help fish that are sick. If you notice that the fish you are caring for is struggling, then it might be time to humanely decapitate them.


Dibyajyoti Bordoloi is the creator and author of FishCampRehab.com, a third-generation experienced fish keeper and owner of a successful pet breeding farm. He is also a member of the Center for Wildlife Rehabilitation And Conservation (Assam), the Marine Aquarium Societies of North East India, and the Kaziranga Nature Conservancy of Assam.

Leave a Comment