Why Do My Fish Keep Dying: 15 Reasons and Solutions

Fish are harder to diagnose than other pets while, arguably, more things can go wrong. Cats, dogs, birds, and lizards at least breathe the same air we do. Their potential ailments and discomforts could be hard to detect, too, but environmental factors are relatively easy to control.

The reasons why fish keep dying range from poor water conditions and improper acclimation protocols to inadequate setup or bad company. Disease, stress, tech failures, overfeeding, overmedicating, and more can also contribute to high mortality rates.

Poor acclimation and transportation are among the chief killers in the aquarium hobby, but let’s examine all the XX reasons why fish keep dying.

We’ll start our investigation with a few common beginner mistakes before moving on to more subtle pointers that can remain undetected even among experienced aquarists.

New Tank Syndrome

New tank syndrome happens the first few weeks after an aquarium is set up. It doesn’t matter whether it is a bare bottom tank, one with inert substrates, or a topsoil setup.

The reason why new tank syndrome appears stems from the filter media, which is still immature and unable to break down the excess waste. Aquarium filters rely on mechanical filtration to capture larger pieces of debris and on biological input from countless invisible bacteria that detoxify the water column. These bacteria need time to appear and grow sufficient colonies.

Many fish shops sell starting bacteria for aquarists who want to shorten the maturation of their new tanks. While helpful, they aren’t really necessary — dropping a bit of fish food in the new tank will help them spawn.

After that, all you need is patience and, ideally, water testing kits to measure ammonia and nitrite levels. Even without those, 4-6 weeks should be more than enough for a healthy nitrogen cycle to establish. You can add plants right from the start, that will only help the process as plants house beneficial bacteria too, and break down toxins

Regardless of whether you have tested the water extensively, always start by adding a few (inexpensive) fish at first and wait a few days to see how they fare. If they stay healthy and exhibit no signs of stress, it would be safe to increase their numbers.

Poor Acclimation

Proper acclimation is crucial for keeping new fish alive. The initial process of introducing new livestock to the tank is relatively simple:

  1. Open the transportation bag.
  2. Let it flow into the tank so that both acquire the same temperature; secure the bag to prevent it from spilling into the tank or filling up with tank water.
  3. Keep it floating for 20 minutes, adding a bit of aquarium water every 4-5 minutes. In the end, the transportation bag should contain at least 65% aquarium water.
  4. Take the transportation bag out, catch the fish, and release them into the aquarium.
  5. Discard the transportation bag and its water.

That’s the most basic acclimation protocol, but you can easily create a drip acclimation setup, which is more hassle-free and safer.

A couple of important pointers to consider:

  1. Quarantine your fish and plants — Putting plants and fish in a quarantine tank greatly reduces the risk of infectious diseases and unwanted snails. After quarantine, fish should be acclimated as described above, though.
  2. Feed your new fish — Fish usually travel with little to no food across borders and on longer distances. In other words, most pet stores receive underfed deliveries. That’s why giving extra food to newly acquired fish allows them to gain the weight they’ve lost, helping their acclimation. It is best done in a hospital tank, as overfeeding your main tank isn’t ideal (more on that in a bit).

Feeding new fish extra for 7-10 days should be considered part of their acclimation.

Buying Bad Fish

Most fish stores make efforts to not sell fish in poor health, but the buyer must be always vigilant when purchasing fish. Beginners can easily miss the most common signs of fish in poor health.

Here is what to look for when purchasing new fish, in a nutshell:

  1. Healthy fins — No tears, splits, or other damage.
  2. Healthy skin or scales
  3. Normal swimming — Glass surfing or swimming in awkward circles indicate either stress or sickness. Lying at the bottom (unless bottom dweller, of course) or general sluggishness compared to other fish of the same kind also indicate problems.
  4. Healthy eyes — White, foggy eyes or any kind of overgrowth (which can happen to certain betta fish) above the eyes should be avoided.
  5. White spots — Can signify ich or another disease. Don’t buy any fish from a tank with even a single individual with obvious sickness.
  6. Healthy gills — Check that the gills look and work normally; increased ammonia levels can lead to gill-burn which isn’t fatal if checked in time, but it is better to not buy fish with such issues.
  7. Healthy anus — Check the anus of the fish for signs of reddening or blood; if you see any, stay away from the entire tank and possibly fish store, as that could indicate worms. Some of them are notoriously hard to remove and highly contagious.

Size, general behavior, levels of activity, and potential signs of aggression are a few other factors to consider when purchasing fish, but recognizing those comes with experience and exposure.

Poor Setup

Putting fish in an inadequate environment will stress them out and eventually kill them. The right temperature and water chemistry are the two main factors that determine whether a fish will thrive or struggle.

Water flow is another big factor. Putting a betta fish in fast-flowing water will tear its fins down, tire it, and kill it soon enough.

The right substrate also helps a lot. Sand is better suited for kuhli loach and corys, whereas goldfish will be OK with a bare bottom tank that is easy to clean.

Hiding spaces also are necessary for certain fish and shrimp.

Light, on the other hand, is much more important for plants than fish. Most species can live in darkness even, but light will regulate their sleep cycle and will improve their colors and feeding. Many fish rely on their eyesight to spot and consume food.

What’s more, you’ll see them much better. Besides its tremendous aesthetic appeal, light will help you detect problems early on. If you have no live plants, any kind of light will do.

Putting the tank very close to a window can lead to temperature fluctuations, too. On a particularly cold night, the heater might be unable to cope. Conversely, when it is hot outside, the water column with also heat up.

Another thing to avoid is placing the tank on top or very close to a TV or a powerful stereo. While fish can be mesmerized by the moving colors of a TV screen, sudden sounds and loud noises will scare them every time.

No Lid

Some basic aquariums are sold without a lid. It is much better to have one because fish can jump out. Even fish that aren’t considered jumpers like guppies or gouramis.

The lack of a lid can lead to unpleasant situations like dragonfly or damsel eggs. Rare, yes, but really not unheard of, especially in rural areas. Their larvae are brutal predators that can decimate your aquarium population.

Damsel Larva

Lastly, lids reduce massively water evaporation, lowering the need for water changes or top-ups. Besides lowering the maintenance labor, stable water levels keep fish happier.

Bad Company

Selecting the right fish species for a community tank can be tricky. Some are quite aggressive, others are “simply” territorial, yet others would attack only certain fish.

Bettas are a classic example. Male individuals can coexist with most bottom dwellers and some tetra species ember tetras or neons, but they will be aggressive toward other colorful, high-swimming fish.

Danios are largely peaceful, but they tend to nip fins and are likely to bother slow-swimming guppies (or bettas) to the point of exhaustion and death.

Small rainbow sharks are peaceful, but once they enter adolescence, they tend to become territorial and aggressive toward other species of similar size and proportions.

Probably the best way to minimize fish bullying (other than isolating the aggressor from the victim in separated tanks) is to provide ample hiding space, but prevention is much more effective. When buying new fish, research how compatible it is with the fish you already have. Ask in social media groups and in the local fish store to ensure maximum compatibility.


Overfeeding is one of the most common mistakes among new fishkeepers. Feeding fish is entertaining and the little swimmers, while not quite at puppy-eyes-levels, beg for food all the time.

Regular overfeeding will harm the water quality of your tank, can make the fish obese, and can ultimately kill them. It can also contribute to pest snails’ populations exploding.

Feed your fish as much as they can eat in 2-5 minutes. It is better to give them less food in two feeding sessions than a lot in one, as uneaten food decays and increases ammonia levels.

Inadequate Water Chemistry

While flow and temperature are relatively easy to regulate, water chemistry is considerably trickier. Every water source has its intrinsic characteristics.

For fish, the most important is the water alkalinity or acidity, measured in pH. General hardness (GH) and carbonate hardness (KH) also contribute to the water quality, but let’s focus on pH first, as most common tropical fish can handle a reasonably wide range of hardness.

(KH and GH become more important when growing live plants, even though they play a role for livestock, too.)

pH testers are widely available. Strips are easier to use, but tests with reactive reagents are more accurate. In any case, unless you are aiming for very specific pH levels, any test would do. Most fish and many shrimp will be happy in the 6.5-8.0 pH range.

Test your aquarium water regularly at first, at the same time of day (pH varies throughout the day) to get a clear idea of its average level. If it is within the 6.5-8.0 range, you’ll be able to keep bettas, corys, danios, and many more pretty fish.

Arguably, the more important thing is to avoid sudden pH swings when conducting water changes. pH shock is a real killer. High KH levels act as a buffer against pH changes.

Water changes (with treated water from a good source), botanicals or water hardeners, more powerful filtration, and fewer fish are the most direct ways to rapidly improve water quality. Chemical solutions exist as well but optimizing setup and maintenance practices are a better call in the long run.

Poor Maintenance Practices

Regular maintenance is a must for newly set up tanks. In general, as tanks mature, the need for maintenance lowers, but that depends on the setup.

High-tech aquariums always require regular maintenance to keep their superb looks and to avoid excessive algae growth. On the other hand, more casual tanks, with thick substrate layer, sufficient plant mass, and adequate filtration mature over time and tend to reach equilibrium.

However, having a reliable maintenance protocol is crucial. Take your time and establish a working routine that allows you to clean the tank thoroughly, without risking the lives of its inhabitants.

Toxins from External Sources

A common way to introduce harmful toxins into the water column is through aquarium maintenance tools. Buckets, sponges, hoses and such should be used exclusively for the fish tank and should never be washed with cleaning chemicals, including soap.

Adding water from the tap can lead to nasty surprises.

As part of a good water maintenance routine, you should always test the water from the tab because some utility companies perform unannounced maintenance of their infrastructure.

Typically, the maintenance involves pumping high amounts of water through the pipes to flush them from debris. This often remains unnoticed by humans, but for days and weeks after such an operation the water chemistry can be completely different.


Diseases of various kinds can kill fish and other aquarium livestock. 

Good quarantine discipline can go a long way in disease prevention, but inadequate diet, low temperature, or polluted tank water can cause a vast number of fish. One of the side effects of stress in fish is a weaker immune system, whereas lower temperature is conducive to common diseases like ich.

In most cases, the symptoms of the sickness will become fairly obvious so you won’t be wondering why your fish keep on dying.

The most common aquarium fish diseases include:

  • Ich
  • Fin rot
  • Dropsy
  • Fungus
  • Worms
  • Pop-eye
  • Swim bladder infection
  • Velvet

The list goes on, but once the disease manifests itself it becomes fairly obvious something’s very wrong. Luckily, most common fish illnesses can be treated (worms are probably the hardest to get rid of).

Still, sometimes drastic measures like tank reset or at least going fallow for about three months may be necessary. Fish medication must be administered with care.


Administering large doses of antibiotics can be dangerous, especially in a community tank. Different species have different tolerance to medication. Kuhli loaches are quite sensitive, for example.

In any case, never surpass the recommended dosage, and don’t mix medications. Use one for as long as it is advised on the package. In case the disease doesn’t respond to the treatment, try another only after the first one has run its course.

Prolonged antibiotic treatment can ruin the colonies of beneficial bacteria that help detoxify the aquarium water, which can lead to ammonia spikes and sudden fish death.

Tech Fails

Aquarium equipment has evolved drastically over the years, but tech fails still happen.

The most common piece of equipment that can kill your fish quickly is the heater. If you are using one and it malfunctions, especially in a smaller tank, the fish can get either too hot or cold. Neither is great.

А broken thermostat may prevent the heater from turning off at the desired temperature.

А broken heating coil will make it completely useless and the water temperature may drop quickly during winter, in a cold room.

Malfunctioning air pumps or filters also should be replaced, but they usually allow for a day or two to react. In very small tanks, the oxygen levels may drop more quickly than you can replace the filter, but a quick water change will offset the issue.

Of course, technological breakdowns are the deadliest when you aren’t around your fish tank. Going on a 4-5 day trip only to return to a cold, lifeless aquarium is one of the worst things a fishkeeper can experience.

Human Error

Keeping strong water change protocols reduces the risk of messing things up in the aquarium, but fishkeeping usually is a long-term hobby. Being around aquariums for years practically guarantees complacency at some point or another, overlooking something in the initial setup, or…

There isn’t an exhaustive list of what can go wrong when keeping fish. Mistakes will happen, the important thing is to remember their lessons, keep them in mind, and not get discouraged by the inevitable fail.

Old Age

Fish have different lifespans. Buying too young fish has the potential to let them live longer in your aquarium, but younger individuals are also more fragile and harder to raise.

If you are going regularly to your local fish store, you will see when they have new fish brought in. As a rule of thumb, most commercially sold fish are fully grown but fairly young. (Of course, large fish like oscars or rainbow sharks often are sold as adolescents.) The business necessities dictate fast turnovers.

If your fish keep dying from old age and old age alone, then you are doing a stellar job!

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