Why Do My Fish Keep Dying? 12 Main Causes & Solutions

    how to save a dying fish

    Have you ever been in the position where you start to question yourself, “why do my fish keep dying”?

    There are various reasons why the fish in your tank may keep on dying. Some of these reasons include overcrowding, diseases/infections, stress, not cleaning the tank enough, and much more. This post will go over many of the common causes, as well as quick solutions.

    Don’t feel like it’s just you! Most fishkeepers have a rough start, especially when there’s little research getting done before doing all the fun stuff (setting-up a tank, aquascaping, etc.).

    Every fish species is special in its own way. Most aquarists get attached to the lively bursts of color and grace that inhabit our fish tanks.

    That’s why it can be really distressing to be faced with several fish deaths in a row, especially after putting in a lot of effort to get an aquarium up and running with all the necessary gear.

    Luckily, most fish deaths are caused by preventable circumstances. There’s nothing you can do about a fish dying of old age, but mass-deaths are always a cause for concern.

    This post will go over 12 of the most common reasons why your fish keep dying and ways you can prevent premature fish deaths.

    Why Do My Fish Keep Dying? 12 Most Common Causes

    The 12 main reasons why your fish keep dying (and their respective solutions) are summarized in this table below:

    Causes Solutions
    New Tank Syndrome Cycle tank before adding fish, feed less, test water, do small water changes daily.
    Wrong Tank Size Move fish to appropriately-sized tank or avoid adding more fish.
    Unsuccessful Transfers Acclimate new fish before releasing them into your aquarium.
    Overcrowding Adjust good bacteria in your tank to new fish additions, stop adding new fish if overcrowded.
    Excessive Water Changes Plan regular water changes every two weeks.
    Poor Water Quality Test water parameters (pH, ammonia/nitrites, salinity, etc.) weekly.
    Sudden Temperature Changes Move aquarium to a draft-free location, away from direct sunlight and heating elements.
    Overfeeding Feed fish a quantity of food that they can finish eating within 2 minutes.
    Not Cleaning The Tank Enough Set-up a regular maintenance schedule.
    Deep-Cleaning A Tank Start over as in an uncycled tank, do small frequent water changes.
    Diseases, Infections Quarantine new fish in a separate tank for 2 to 4 weeks.
    Stress Keep water quality and tank conditions within acceptable ranges, choose right tank mates.

    Keep reading for more in-depth information!

    1) New Tank Syndrome

    New tank syndrome” is what fishkeepers usually have to deal with when first setting up a new aquarium if they don’t allow it to get cycled before adding fish into it.

    The culture of nitrifying bacteria (the good bacteria) hasn’t had time to settle, grow, or get established, so any fish introduced in the said tank will most likely die of ammonia poisoning without warning.

    Fish waste (fish food, plant matter, etc.) will break down into ammonia/nitrites in any tank, but in a tank that has well-established good bacteria, these toxic chemicals get turned into non-toxic nitrates.

    An aquarium that’s still in the “new tank syndrome” phase will simply not have enough good bacteria to keep water quality good enough to be compatible with life for your pet fish.


    If you already have fish dying in a newly set-up tank that hasn’t been cycled, here are some steps to take:

    • Don’t add any more fish to the tank.
    • Feed fish less than you normally would to reduce chances of leftover food turning into more ammonia.
    • Test ammonia/nitrite levels often using aquarium test kits.
    • Do small water changes daily until water parameters stabilize.
    • Add a bacteria starter into the water to speed up the tank cycling process.

    2) Choosing The Wrong Tank Size

    what to do when your fish is dying
    Endler’s livebearer (Poecilia wingei)

    The appropriate tank size for your set-up has less to do with where you’re planning to keep it and more to do with the number and type of fish that you’re planning on keeping in it.

    Having large active fish in a tank that’s too small for them to have open swimming areas can stress out fish to the point of death. A bit dramatic, but it can happen, as stress makes fish vulnerable to diseases and premature deaths.

    Getting the wrong-sized tank usually happens when a fishkeeper buys pet fish as juveniles and doesn’t do research on how big those fish can grow and what their tank set-up needs are.

    A tank that is too small will also show increases in pollutants faster. When it comes to water quality, larger tanks are easier to maintain. Simply because it takes more of a toxic agent to cause a measurable change!


    If you started questioning why the fish in your tank keeps dying after having some gorgeous, gentle giants dying in your tank for no apparent reason, there are two quick measures you can take:

    • There may be fish of the same size in your tank as those previous fish that have already died. If you think this might be the reason why you’re losing fish, moving them to an appropriately-sized tank is the safest route to take.
    • If losing some of the pet fish in the tank freed up enough space to make life comfortable for the remaining fish, avoid adding any more fish to said tank.

    3) Unsuccessful Transfers

    Having a new fish die soon after you introduce it into your tank can be the result of an unsuccessful transfer.

    Along with the stress of being put in a bag and going on a wild ride, there are plenty of things that can go wrong during a transfer.

    Not acclimating new fish to your tank’s particular water conditions (temperature in particular) can cause even the hardiest fish species to succumb to death shortly after a transfer.

    Small or fancy fish with long flowy fins are incredibly sensitive to transfers. Unfortunately, they can easily get injured even when trying to get them into a fishnet.


    • Don’t add too many new fish to a tank at the same time. The sudden influx of fish waste can make your water quality drop significantly.
    • Acclimate new fish before releasing them into your aquarium. Here’s a video explaining the acclimation process:
    YouTube player

    4) Overcrowding

    Overcrowding usually happens in community tanks, especially if you’re new to the hobby and you’re enthusiastically adding more and more fish to an aquarium, without letting the nitrifying bacteria in your tank catch up to the sudden spike of waste influx. 

    Ammonia poisoning is more likely to happen when there are simply too many fish in a tank that’s too small to accommodate them all. This can easily happen with smaller fish species, not just large and messy ones.

    Overcrowding and fish never having refuge areas to hide in when stressed can be a recipe for disaster and a big reason why your fish keep dying.


    • Allow the culture of good bacteria in your tank to adjust to any new fish additions to a tank. Space out introducing new fish in at least 4-week intervals.
    • If your fish show signs of distress, and you realize you’ve possibly overcrowded the tank, stop adding new fish to the tank.
    • Move fish that need lots of room to swim, and grow, into a separate tank.
    • If you don’t have a second tank to move some of the fish into, add decorations/tall aquatic plants to give fish nervous enough hiding spots and reduce stress levels.

    5) Excessive Water Changes

    Paradise Fish (Macropodus opercularis)

    If a fishkeeper doesn’t stay on top of a periodic tank cleaning routine, water inside a tank is bound to become cloudy, yellow, and discolored.

    That’s when inexperienced aquarists will do a water change of 70% or more and will effectively send their aquarium into “new tank syndrome,” as most nitrifying bacteria in the tank is lost.

    This type of excessive water change can send your tank into shock. Sudden and significant changes in water temperature, pH level, salinity, and sometimes chloride levels (if tap water hasn’t been allowed to stand for 48 hours) will cause massive fish die-offs.

    In a well-established tank that gets regular water changes, fish will rarely be faced with unstable water parameters.


    • If the deed is done, and you’ve had a significant number of fish die, act as if your tank is back into cycling mode. Test the water daily, add nitrifying bacteria to the tank, and do small and frequent water changes (10%) until water parameters stabilize.
    • Plan regular water changes to avoid this happening again. Doing 15-25% water changes every two weeks is enough to keep your water looking clean, and to avoid nitrate levels from rising dangerously high.
    • Avoid adding any new fish into the tank until the culture of good bacteria gets re-established.

    6) Water Quality & Unstable Water Parameters

    As the water in your aquarium is the enclosed environment that sustains your fish, you can only imagine how important maintaining optimal water quality is to keeping fish disease-free and, well, alive!

    There are plenty of factors that can cause water quality to drop, and most of them have more to do with unstable water parameters than anything else.

    If you haven’t been testing the tank’s water with aquarium test kits, there’s probably a lot of guesswork going into figuring out why your fish keep dying.

    Most aquarium fish have a pretty set range of water parameters they prefer (and need to survive!), and crystal-clear water is not the most reliable indicator of water quality in a tank.


    • Test water parameters (pH, ammonia/nitrites, salinity, water hardness, Mg, Ca, etc.) weekly.
    • Rather than trying to chase the perfect pH level (or any other parameter), try to keep levels stable, within an acceptable range for the fish species that you have living in the tank.
    • Act fast when there are clear warning signs (high ammonia level spikes!) to prevent more pet fish deaths. This may include doing an immediate 50% water change and then closely monitoring ammonia/nitrite levels until water quality stabilizes.

    7) Sudden Changes In Water Temperature

    Red-spotted killi (Aphyosemion cognatum)

    Some fish species are hardier than others, but most fish can only tolerate a narrow range of temperature water. Quick temperature swings can put fish at risk for diseases, stress-overload, and death.

    Water temperature changes inside an aquarium can occur fairly easily when:

    • Having a tank in direct sunlight
    • Keeping the tank next to a heater/cooling vent
    • Overheating by keeping the aquarium light on too long
    • Sudden cool-downs via a drafty spot in your home near your tank
    • A malfunctioning water heater is going unnoticed.

    If you’ve had several pet fish deaths, and any of the above sounds similar to your tank’s set-up, this might be the reason why your fish keep dying.


    • Move your aquarium to a draft-free location, away from direct sunlight exposure and any heating elements.
    • Check to see if water temperature fluctuates depending on whether your tank lights are on or off.
    • Make sure the water you use when performing water changes is at a temperature that your fish are accustomed to.
    • Check that your heater is working properly on a regular basis. A submersible digital thermometer will help you monitor water temperature to see if the heater is efficient for your tank size and set-up.

    8) Overfeeding

    Overfeeding fish is the most common culprit when it comes to sudden massive deaths in an aquarium.

    Apart from being harmful to greedy eaters, it can also throw your tank into bacterial imbalance.

    Greedy fish will overeat and beg for more food. Then they end up constipated with swollen bellies, and in a great deal of distress.

    Food that doesn’t get eaten at the beginning of a feeding session will sink to the bottom of the tank. Then they will start decomposing into ammonia/nitrites.

    A small amount of leftover food can usually be handled by the tank’s culture of nitrifying bacteria, but overfeeding too often will more than likely turn your tank’s water toxic and kill fish left and right.


    • Feed fish a quantity of food that they can finish eating within 2 minutes. Do this once or twice a day for adult fish and up to 4 times per day for fish fry.
    • Siphon your tank’s gravel periodically to remove some of the leftover food.
    • Check and clean the aquarium’s filtering system often. Uneaten food will build-up on filter media and cause the filter to perform poorly.
    • Consider adding a bottom-feeder to your community tank. They can act as your own personal cleaning crew, lowering chances of overfeeding, leading to ammonia poisoning.

    9) Not Cleaning The Tank Enough

    electric yellow cichlid
    Electric yellow cichlid (Labidochromis caeruleus)

    Obviously, neglecting your tank to a point where you would be better off starting over will put all the fish in your tank at risk of dying for a myriad of reasons.

    Smaller tanks, in particular, will start to become toxic environments for fish if water quality drops dangerously low. This is also true when ammonia, nitrite, and nitrate levels spike. Algae overgrowth can also cause problems.

    In a properly cycled tank, ammonia and nitrite levels should stay close to 0 ppm. But nitrate will concentrate if you don’t perform water changes or have plants to consume it. Most fish tolerate levels of 10-20 ppm. But the lower it tests, the better.


    If you just learned the hard way that slacking off on tank cleaning duties will wipe-out fish in your tank quickly, it’s time to set-up a maintenance schedule that you can follow through with.

    Doing smaller clean-ups regularly will be easier on you and a lot healthier for aquarium fish. 

    Here are some of the basic tank hygiene tasks you need to do on a regular basis to keep an aquarium safe (and compatible with life!) for your fish:

    • Water changes – every two weeks (20%)
    • Clean filter – monthly
    • Change/clean filter media – monthly
    • Change biological filter media – every 3 to 6 months
    • Siphon gravel – weekly
    • Use test kits – weekly
    • Clean light fixtures/exterior glass panels – monthly
    • Clean interior glass panels – bimonthly (using an algae scrubber)

    10) Deep-Cleaning A Tank

    Even if you find yourself with a neglected tank on the verge of a biological breakdown, know that deep-cleaning an aquarium can actually cause more fish deaths than gradually bringing a tank’s conditions back within optimal ranges.

    Deep-cleaning a tank, rather than following a regular maintenance schedule, will kill off the culture of good bacteria in your tank, leaving it without its vital biological filtration system.

    Rinsing decorations under tap water, washing gravel, doing a huge water change, changing all filter media all at once, etc., will make your freshly deep-cleaned tank a ticking time-bomb for your fish, the same as an uncycled new tank would be.


    • If you’ve already deep-cleaned a tank, start over as you would do in an uncycled tank. Do small frequent water changes, and only clean build-up in your filtering system for a while.
    • To avoid getting into this same situation going forward, perform tank maintenance tasks periodically. Alternating which “spot” you’re cleaning to give good bacteria a chance to re-establish between cleanings.

    11) Diseases, Fungal Infections, And Bacterial Infections

    aquarium fish died overnight
    Blue ram cichlid (Mikrogeophagus ramirezi)

    Parasites, fish-killing bacteria, and various diseases can make their way into your fish tank without you noticing. It can happen whenever you introduce something new into your aquarium.

    Recently bought fish can carry diseases without showing any obvious signs. Live aquatic plants can introduce hitchhiking parasites into the tank. Some improperly-washed décor elements can carry-in harmful bacteria.

    Even forgetting to sterilize fishnets and aquascaping tools after each use can be the reason behind diseases wreaking havoc among your tank’s fish population all of a sudden.


    • Quarantine new fish in a separate tank for 2 to 4 weeks before adding them to your community tank. This will allow you to monitor your new additions and see if they get sickly. You can also dose medicine and save fish from diseases in the same quarantine tank without putting your other fish at risk.
    • Try to thoroughly clean new aquatic plants in old tank water before planting them in your aquarium. Do the same with any new decorations.
    • Keep a container of disinfecting solution on hand to sanitize fishnets and aquascaping tools after every use.
    • Never add aquarium water from the pet store to your fish tank. It is always loaded with bacteria, parasites, and viruses. Pet store fish live in close proximity and are always under stress. The perfect breeding ground for germs.

    12) Stress

    This is the most common reason for substantial die-offs in an aquarium. Stress in fish can be brought on by countless external factors, such as:

    • A drop in water quality
    • Unstable water conditions
    • Overcrowding
    • Incompatible fish species being paired up
    • Stress caused by environmental factors: loud noises, tapping the glass, constant sudden movements


    • Keep water quality and tank conditions within the acceptable ranges that your fish are comfortable with
    • Choose the right tank mates for the fish you’re housing
    • Add aquatic plants and plenty of hiding spots for nervous fish
    • Move fin nippers, bullies, and aggressive fish from the tank if they’re obviously stressing out their tank-mates

    FAQs About Why Fish Keep Dying In An Aquarium

    Betta fish (Betta splendens)

    How do I stop my fish from dying?

    Test the water regularly and do partial water changes to keep ammonia and nitrites from building up. Also make sure the tank isn’t overcrowded, give them places to hide, and feed a varied diet. Keep a close eye for signs of disease too.

    Why do my fish keep dying in my tank?

    There’s a few usual suspects – poor water quality, not enough filtration, aggression from tankmates, sharp decor injuring them, rapid changes in water parameters, or introducing new sick fish without quarantine. It helps to test for ammonia and nitrites and fix any issues.

    What is the main cause of fish dying?

    Water quality is the number one thing. Ammonia and nitrite spikes from overfeeding or waste buildup are deadly. Also watch for fungal infections, aggression, lack of oxygen, rapid pH shifts, chlorine, and improperly acclimating new fish.

    Why is my fish dying but my water tests perfect?

    Even with good parameters, fish can fall ill from introducing disease, bullying tankmates, nutritional issues, or environmental stress. Quarantine new fish for a couple of weeks and watch them closely. Remove aggressors.

    Should I change water after fish died?

    Absolutely – take the dead fish out right away and change at least 25% of the water. This dilutes any ammonia or toxins released when it died. Test the water too in case there’s an underlying issue you need to tackle.

    When a fish dies does it sink or float?

    At first it’ll sink but as bacteria build up gases inside, it’ll float to the top within a day or so. It’s best to remove it quickly so it doesn’t foul up the tank water as it decomposes.

    What do fish do when they are dying?

    They may stop eating, isolate themselves, lay on the bottom breathing heavily, rub against objects, lose balance, or clamp their fins tightly to their body. If you see this, test the water and treat any issues immediately.

    Why is my fish not moving but still alive?

    A very lethargic, still fish that’s still breathing is seriously ill. This could be from disease, bullying, poor water quality, lack of oxygen, or shock from a big temp change. Test and change the water first, isolate it if needed.

    Where do you put a dead fish?

    You can bag it up and toss it in the regular garbage outside, just don’t flush it. For smaller fish, you can bury them in your garden. Just be sure to contain the body so you don’t transmit disease.

    What kills fish in a tank?

    Spikes in ammonia or nitrites, low oxygen, pH swings, chlorine, aggression between fish, undersized tanks, sharp decor, disease outbreaks, old age, thermal shock, or overfeeding can all be fatal. Stability is key!


    As you can tell, most mass-deaths in a fish tank are caused by “user-error.” This is either by mistake or by just not having enough experience with fishkeeping.

    A fish’s death all of a sudden is how you learn the hard way, but don’t get discouraged!

    Instead of just asking yourself why your fish are dying without being proactive, doing some research (like finding this article!) will help you put an end to preventable fish deaths.

    Monitor your pet fish and their water conditions regularly. Also, stay on top of your tank cleaning routine and take proper care of your aquatic friends. If you do these, you’ll be rewarded with happy, healthy fish!

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